Use PathBuf instead of String where appropiate

[jonasbb]

CratePreludeData.crate_root and ExternalCrateData.file_name both store file paths, so using PathBuf as the type would semantically make more sense.

[nrc]

iirc there was some issues with serialising these types

[Xanewok]

These are serialized as an array of bytes in JSON, and is used now for SpanData::file_name.
Is the representation something that would require 'fixing' on the rustc-serialize side?
I assume it's encoded as bytes to handle platform differences with path/string representations but it seems to be working now.
Should we, then, avoid using PathBuf with rustc-serialize altogether or change some Strings to PathBufs and possibly change the JSON representation?

[kamalmarhubi]

Assuming that #15 is accepted, moving to serde with custom serialize_with / deserialize_with becomes an option.