This releasae catches up from the main ANTLR repo in order to

finally make the 4.13.1 release. It 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 exprimental 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 v4.13.0 of the ANTLR4 go runtime

Release v4.12.0 of the ANTLR4 go runtime