internals: optimize std::unordered_map internals with noexcept

[Skylion007]

Description

• Updates various STL specialization function with noexcept. This allows STL to optimize the hashing in unordered_maps. Otherwise, these has values are stored in the map instead of recomputed which adds memory and hurts locality. Surprisingly, this change is fully ABI compatible too since the noexcept signature is erased before it's mangled.

Suggested changelog entry:

• optimize pybind11 internal hash functions for STL maps.