bip_metainfo: API Type Changes

[GGist]

Will have to double check that we aren't breaking spec, but in general, having to deal with the current types has been painful in downstream libraries (like bip_disk).

• File::length() should return a u64
• InfoDictionary::piece_length() should return a u64

On a not spec breaking related note:

• File::paths() should return a &Path (so we should allocate a PathBuf internally)

[GGist]

Closed 3794dbb