✨ Add indexing for type_map

[elbeno]

Problem:

• Having to type out stdx::type_lookup_t<M, K> (or any of its variations for type-value combinations) is cumbersome, especially if you have a nested map type.

Solution:

• Provide operator[] for looking up types and operator() for looking up values. (Providing both in one operator would be much more complex, if at all possible/nice.)

Note:

• In the case where an incomplete type is the key or value in a type_map, the argument/return types can be type_identity<T>.
• Because operator() takes a (non-constexpr) argument, conventional use is to pass this as an integral_constant or cts_t: something that exposes a constexpr static value.