(fs) Optimize UnixPath.encode implementation

UnixPath.encode duplicates the logic exposed via JavaLangAccess.getBytesNoRepl, just with rewrapping any exception thrown in a InvalidPathException. Merging this removes a commonly created ThreadLocal, and will likely hit one of the fast-paths in StringCoding.encode.

The toCharArray done for the purpose dealing with paths that need normalization on Mac is also costly up front in a way that could be avoided.

This was found when looking into startup profiles of the Spring PetClinic.