Improved path utilities

It was silly of me to omit underscore prefixes when naming `sprinter_prepend_path`. By using the underscores, it helps keep a function that should only be reused by sprinter-based code out of the easily-accessed shell autocomplete (and competing with `sprinter` itself).

`__sprinter_remove_path` is designed to aid internal tools that are incorrectly removing temporary values from the `PATH` env var.