Unify position typedefs.

AudioSource::MicrosecondPosition, PlayerPosition::Unit, and
TimeParser::Time are all now TimeParser::MicrosecondPosition.
[Rebase!] Actually unify position typedefs.
Fix typo in TimeParser test.
Remove long-form units.

As per discussion with @LordAro.
Clean up Multiplier.

Fix Doxygen comments, and use our new-found horizontal space to make the
Multiplier members have decipherable names.
[Rebase!] Fix tests.