Make indexes not arch-specific

The data in the index isn't inherently arch-specific in any way, and
writing the data structures in a portable format doesn't actually make
the reading and writing code much more complex, so there's no real
reason not to.

Closes #120.
Fix unimportant typo in comment
Don't write fields which don't apply to the current track type to the index

Bumps about 10% off the compressed size of the index of a totally
arbitary file I indexed.