Release 4.13.1

The 4.13.1 release has:

• Some minor performance improvements
• An optional build tag -tags antlr.nomutex which allows a build without
  using mutexes, when the user knows they will not use it with multiple
  go routines. (Note, somewhat experimental)

Note that I did not remove the sort.Slices calls. They will not be experimental for long
and are implemented efficently. Please make a fork if you must use it without this
dependency. Maybe review whether you need to do so for the sake of one call.

Release 4.13.0

This is a big release, hence the minor version bump.

See documentation at: https://github.com/antlr/antlr4/blob/master/doc/index.md for complete details.

Release 4.12.0

Release 4.12.0 is the first release to the new ANTLR Go repo.