Reject integral keys for contains(), find(), and count() at compile time (#5705)

j.contains(0), j.find(0), and j.count(0) used to compile: the literal 0 is
a null pointer constant, so it converts to a null const char*, and the
overloads taking const typename object_t::key_type& accepted it by
constructing a std::string from that null pointer, which is undefined
behavior (a crash with both libc++ and libstdc++). value(0, default_value)
had the same problem in C++11, where the object comparator is not
transparent.

Add deleted overloads for integral arguments to contains(), find()
(const and non-const), count(), and value() so that these calls are
compile errors in every supported language mode instead of crashing.
Calls with string, string_view, json_pointer, and size-typed element
access (at(), operator[](), erase()) are unaffected.

Fixes #5657.

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

docs/mkdocs/docs/api/basic_json/contains.md

@@ -58,6 +58,10 @@ Logarithmic in the size of the JSON object.
 
 - This method always returns `#!cpp false` when executed on a JSON type that is not an object.
 - This method can be executed on any JSON value type.
+- Calling this function with an integer argument (for example, `#!cpp contains(0)`) does not compile: such an argument
+  would otherwise implicitly convert to a null `#!cpp const char*` and, from there, cause undefined behavior when
+  constructing a `#!cpp std::string` for the object key. To check for an array element instead, use [`at`](at.md),
+  [`operator[]`](operator%5B%5D.md), or compare against [`size`](size.md).
 
 !!! info "Postconditions"
 
@@ -117,3 +121,5 @@ Logarithmic in the size of the JSON object.
 1. Added in version 3.11.0.
 2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
 3. Added in version 3.7.0.
+4. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of
+   causing undefined behavior at runtime.