refactor: reduce overloads with _mini_ntmbs

A small wrapper type inspired by string_view, but refers to a
null-terminated source, and can be used as a drop-in replacement
to `char const*` in implementation.
fix(msvc): synthesized move not supported in VC12
feat(header_dict): initializer_list support

Construct and assign to a header_dict with 2-level braced-init syntax
like what std::map does.

For MSVC, requires >= VC12.