Normalize -0.0 in number hashes and test range ends

operator== compares numbers exactly since #5459, so equal numbers
share one value and convert to the same number_float_t. Update the
comment accordingly, map -0.0 to 0.0 before hashing (std::hash need
not do that), and test -0.0 and the ends of the integer ranges. Show
hash(0.0) in the docs example and note the change in the version
history.

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

docs/mkdocs/docs/api/basic_json/std_hash.md

@@ -37,3 +37,5 @@ whether they are stored as `#!cpp 0`, `#!cpp 0U`, or `#!cpp 0.0`.
 
 - Added in version 1.0.0.
 - Extended for arbitrary basic_json types in version 3.10.5.
+- Numbers that compare equal hash equally since version 3.13.0; before, `#!cpp 0`, `#!cpp 0U`, and `#!cpp 0.0` had
+  different hash values.

docs/mkdocs/docs/examples/std_hash.cpp

@@ -11,6 +11,7 @@ int main()
               << "hash(false) = " << std::hash<json> {}(json(false)) << '\n'
               << "hash(0) = " << std::hash<json> {}(json(0)) << '\n'
               << "hash(0U) = " << std::hash<json> {}(json(0U)) << '\n'
+              << "hash(0.0) = " << std::hash<json> {}(json(0.0)) << '\n'
               << "hash(\"\") = " << std::hash<json> {}(json("")) << '\n'
               << "hash({}) = " << std::hash<json> {}(json::object()) << '\n'
               << "hash([]) = " << std::hash<json> {}(json::array()) << '\n'

docs/mkdocs/docs/examples/std_hash.output

@@ -2,6 +2,7 @@ hash(null) = 2654435769
 hash(false) = 2654436030
 hash(0) = 2654436221
 hash(0U) = 2654436221
+hash(0.0) = 2654436221
 hash("") = 11160318156688833227
 hash({}) = 2654435832
 hash([]) = 2654435899