Refactor src/path/fs predicates.

[epage]

I realized, we could reuse these predicates for being able to create file-backed eq predicates for strings / bytes.

Other changes

• Add find_case to it
• Switch the equality check to use difference.

[epage]

Realized I could get this in now without disrupting my big PR or breaking compatibility by just adding additional impls.

I'll need to remember when I add find_case support for use difference.

[epage]

Just kicked off cargo publish for v0.5.2