Remove frozen as dependency

[mwalcott3]

We already use our own constexpr perfect hasmap when keys are bellow 20. We just need to add a space efficient implementation that can be used for larger numbers of keys.

[stephenberry]

#67 removed frozen as a dependency by including what we use within the glz namespace and removing macros. This allows us to tailor the code to our use cases and make the build process more straightforward.