Address the review of the json_view object index and SIMD scan

From gregmarr's review of #5628:

- Docs and comments say that the hash index is one of the object's keys.
- object_index.hpp: conditional_static_cast for the hash, "slot" for the
  table position, no flag for a degenerate table, and a static_assert
  that the smallest indexed object is larger than the probe window.
- scan.hpp unrolls with NLOHMANN_VIEW_REPEAT_FIRST_8/SECOND_8.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/at.md

@@ -77,8 +77,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
    another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content.
-   Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
-   on average.
+   Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
+   constant time on average.
 2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
    index (unlike `BasicJsonType`'s array, which is random-access).
 3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at

docs/mkdocs/docs/api/basic_json_view/contains.md

@@ -35,8 +35,8 @@ No-throw guarantee: this function never throws exceptions.
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
    another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content.
-   Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
-   on average.
+   Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
+   constant time on average.
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
    that level or the index into the array -- as for [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) with a JSON pointer.

docs/mkdocs/docs/api/basic_json_view/count.md

@@ -28,7 +28,8 @@ No-throw guarantee: this function never throws exceptions.
 Linear in the number of members: unlike the lookup functions, `count()` does not stop at the first match, because it
 has to count every member with the key. Members are compared one after another, in document order. Each comparison
 first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its
-content.
+content. The hash index of the keys of objects with 128 or more members does not help here, since every member
+is visited.
 
 ## Notes
 

docs/mkdocs/docs/api/basic_json_view/find.md

@@ -28,8 +28,8 @@ No-throw guarantee: this function never throws exceptions.
 Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after another,
 in document order, stopping at the first match. Each comparison first checks the key's length
 -- already known from the index, without reading the key bytes -- before comparing its content.
-Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time on
-average.
+Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
+constant time on average.
 
 ## Notes
 

docs/mkdocs/docs/api/basic_json_view/operator[].md

@@ -83,8 +83,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
    another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a
    key of a different length than `key` is rejected without touching the source text.
-   Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
-   on average.
+   Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
+   constant time on average.
 2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
    index (unlike `BasicJsonType`'s array, which is random-access).
 3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
@@ -116,7 +116,7 @@ document.
     member in order and so keep the *last* value for a repeated key, so `#!cpp v["a"]` and
     `#!cpp v.materialize()["a"]` can differ. To get the value `parse()` would give, use
     [`materialize()`](materialize.md) or iterate the members with [`items()`](items.md) and keep the last match.
-    The hash index of a larger object (128 members or more) leads to the first member of a key as well.
+    The key hash index of a larger object (128 members or more) leads to the first member of a key as well.
     [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
     duplicates, in document order. See [Duplicate keys](../../features/json_view.md#duplicate-keys) and
     [`size()`](size.md#notes).

docs/mkdocs/docs/api/basic_json_view/value.md

@@ -70,8 +70,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 1. Linear in the number of members: as for [`operator[]`](operator[].md#complexity), members are compared one after
    another, in document order, stopping at the first match. Plus the complexity of converting
    the found member to `T` (see [`get`](get.md)).
-   Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
-   on average.
+   Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
+   constant time on average.
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
    that level or the index into the array -- as for the [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) overloads that take a JSON pointer. Plus the complexity of converting the resolved value