Keep the last of duplicate keys in the object hash index

Lookups in objects with 128 members or more now return the last member of
a repeated key, like the linear search of smaller objects and like
materialize() and basic_json::parse(). build_object_index let the first
occurrence win, so the same text gave different results depending on the
size of the object.

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

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

@@ -114,8 +114,9 @@ document.
     the *last* member with that key. This is the member [`materialize()`](materialize.md) (and
     [`BasicJsonType::parse()`](../basic_json/parse.md)) keeps, so a lookup in the view and in the materialized value
     agree. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
-    duplicates, in document order. A lookup scans all members for this: it cannot stop at the first match. See the
-    example below and [`size()`](size.md#notes).
+    duplicates, in document order. A lookup in an object without a hash index scans all members for this: it cannot
+    stop at the first match. The hash index of a larger object (128 members or more) leads to the last member of a key
+    as well. See the example below and [`size()`](size.md#notes).
 
 !!! info "JSON pointer resolution"
 

docs/mkdocs/docs/features/json_view.md

@@ -142,7 +142,8 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
   [`contains`](../api/basic_json_view/contains.md), and [`count`](../api/basic_json_view/count.md) resolve to the
   *last* occurrence -- the one `basic_json::parse()` (and so
   [`materialize()`](../api/basic_json_view/materialize.md)) keeps for a repeated key -- which makes a lookup scan all
-  members instead of stopping at a match.
+  members instead of stopping at a match (objects with 128 members or more get a hash index that leads to the last
+  occurrence directly).
   See the [Notes on duplicate keys](../api/basic_json_view/operator%5B%5D.md#notes) of `operator[]`.
 - **No [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) path.** Exceptions thrown by `basic_json_view`'s own
   element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the

docs/mkdocs/docs/home/architecture.md

@@ -215,8 +215,8 @@ packet-beta
 - **Large objects** (128 members or more) get a hash index after parsing
   ([`detail/view/object_index.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/object_index.hpp)):
   an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does
-  not compare every key. The object's `extra` holds the number of its table. Only 65,535 tables fit into `extra`;
-  objects beyond them are searched linearly.
+  not compare every key. Of duplicate keys the table leads to the last, as a linear search does. The object's `extra`
+  holds the number of its table. Only 65,535 tables fit into `extra`; objects beyond them are searched linearly.
 
 For example, `#!json {"a": [1, 2.5]}` becomes five nodes. Each node's elements follow it, and `next` leads from an
 array or object past its subtree: