Return the first member of duplicate keys from lookups again

Looking up the last member of a duplicate key cannot stop at a match, so
every lookup scanned the whole object (1.6 to 3.4 times slower for small
objects). operator[](key), at, find, value, contains, count, and JSON
pointer resolution return the first member again, as yyjson and simdjson
do; materialize() and get<map>() keep the last value, as parse().

The documentation says so in the feature page and on each lookup page,
and explains how to get the value parse() would give. The integer index
templates and the discarded chaining of operator[] stay.

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

docs/mkdocs/docs/api/basic_json_view/items.md

@@ -48,7 +48,7 @@ Constant.
 
 As for [`begin()`](begin.md)/[`end()`](end.md), `items()` visits **every** member of an object, including all
 occurrences of a duplicate key -- unlike [`operator[]`](operator[].md), [`at`](at.md), [`find`](find.md),
-[`contains`](contains.md), and [`count`](count.md), which resolve to the *last* member with a given key. See the
+[`contains`](contains.md), and [`count`](count.md), which resolve to the *first* member with a given key. See the
 [Notes on duplicate keys](operator[].md#notes) of `operator[]`.
 
 !!! danger "Lifetime issues"
@@ -63,8 +63,8 @@ occurrences of a duplicate key -- unlike [`operator[]`](operator[].md), [`at`](a
 
     The example below shows a settings object whose source text records every update to a key as a duplicate
     member, in the order they happened. `items()` walks all of them, so the update history is visible, while
-    [`operator[]`](operator[].md) sees the *last* one, and so does [`materialize()`](materialize.md) -- like
-    [`BasicJsonType::parse()`](../basic_json/parse.md).
+    [`operator[]`](operator[].md) only ever sees the *first* one and [`materialize()`](materialize.md) -- like
+    [`BasicJsonType::parse()`](../basic_json/parse.md) -- keeps only the *last*.
 
     ```cpp
     --8<-- "examples/basic_json_view__items.cpp"