Adds ability for users to specify a custom map class.

A user might want this for performance:

• frozen has benchmarks showing better performance than std::map in most cases
• Per-project custom mapscan be much faster if the input is fully known at compile time
• Running in a constexpr context is frequently used for testing to identify UB in safety critical systems (frozen is fully constexpr)

Changeset includes:

• Macros in the config header file which import the correct map and set types and alias them to something common
• Changes usage of map classes and sets to be the common alias