Polish the macro interface

[yihozhang]

This PR does a few small things:

• Make parsing-related functions public and make them member functions of Parser
• Make the macro type a trait- the current macro type is defined as a fn, which has to be static (e.g., no captured values from the environment nor dynamic behavior)
• Add a few convenience methods so it's usable from outside the crate

[codspeed-hq]

CodSpeed Performance Report

Merging #506 will not alter performance

_{Comparing yihozhang-polish-macros (63520b9) with main (2dfd4f3)}

Summary

✅ 9 untouched benchmarks

[Alex-Fischman]

LGTM