seralize Path/PathBuf as &str/String for wasm32

[ghost]

Hopefully a sensible strategy to get wasm32 to work for those crates that still depend on rustc-serialize.

related:

• https://github.com/rustwasm/team/issues/96
• https://github.com/rust-lang-deprecated/rustc-serialize/pull/190

[CryZe]

There is already an outstanding PR. However this crate is deprecated so it won't get any more releases. The idea is to move away from rustc-serialize instead. I disagree with that though, as it's still one of the most downloaded crates and lots of crates due to semver still depend on it.

[CryZe]

Your solution doesn't compile on wasm32-*-emscripten btw.