Reimplement and bugfix some path functions

[trgwii]

Description

• Get rid of some buffer overruns
• make more functions return more sensible things on NULL arguments
• properly DOCUMENT each function
• Add const type qualifiers where applicable
• Replace some string functions and loops with memcpy

This fixed some broken tests when I cloned with a "." in my parent path (like /github.com/metacall/core/...)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Documentation update

Checklist:

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation (code comments).
• My changes generate no new warnings.
• I have added tests/screenshots (if any) that prove my fix is effective or that my feature works.
• I have tested the tests implicated (if any) by my own code and they pass (make test or ctest -VV -R <test-name>).
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