Expose tag parsing internals

[Serial-ATA]

It may not always be convenient to read an entire file. At this point, the tag parsing internals change infrequently enough that it'd probably be safe to expose them.

It'd be nice to make all of the parse functions have a similar signature to parse_id3v2: https://github.com/Serial-ATA/lofty-rs/blob/6e555d1cb668737c629d0e76b12883532e8bdbce/src/id3/v2/read.rs#L10-L17

Some still have unnecessary Seek bounds or are just too clunky to use externally in their current state (like VorbisComments).