🎨 Allow ct_strings in type_map

[elbeno]

Problem:

• type_map does not easily allow pairs containing ct_string. This doesn't work:

using M = type_map<vt_pair<"A", X>, vt_pair<"B", Y>>;

• type_map returns void or 0 on a miss, either of which might be valid entries.

Solution:

• Allow ct_strings to be used as keys, values and defaults in type_map.
• On a miss, return a missing_t type (or value of that type) by default.

Note:

• This is a minor breaking change: downstream functions that check for void on a map lookup miss need to update. e.g. https://github.com/intel/compile-time-init-build/blob/main/include/interrupt/manager.hpp#L68