Redesign `path` crate to follow `std::path` (#1061)

* The `path` crate now exports a `Path` and `PathBuf` struct,
  akin to `std::path`. This makes it easier to support `std`,
  but also removes unnecessary allocations throughout Theseus.

* Notably, `Path::file_stem()` now returns an `Option<&str>`.
  * This causes `crate_name_from_path()` to return a `Result`,
    which is responsible for most of the logic changes required.

Signed-off-by: Klimenty Tsoutsman <klim@tsoutsman.com>