Correct hash behavior for floating point numbers

This fixes HashMap where a key or part of a key is a floating point
number. To fix this the following has been done:

* HashMap now takes an extra template argument Comparator. This class
gets used to compare keys. The default Comperator now works correctly
for common types and floating point numbets.

* Variant implements ::hash_compare() now. This function implements
nan-safe comparison for all types with components that contain floating
point numbers.

* Variant now has a VariantComparator which uses Variant::hash_compare()
safely compare floating point components of variant's types.

* The hash functions for floating point numbers will now normalize NaN
values so that all floating point numbers that are NaN hash to the same
value.

C++ module writers that want to use HashMap internally in their modules
can now also safeguard against this crash by defining their on
Comperator class that safely compares their types.

GDScript users, or writers of modules that don't use HashMap internally
in their modules don't need to do anything.

This fixes #7354 and fixes #6947.

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