Adds a new latch unit generator with passing control rate tests.
Still need to write audio rate tests.
Fixes latch ugen at audio rate and adds passing unit tests.
Adds a playground demo for the latch unit generator.
Merge branch 'latch'

* latch:
  Adds a playground demo for the latch unit generator.
  Fixes latch ugen at audio rate and adds passing unit tests.
  Adds a new latch unit generator with passing control rate tests. Still need to write audio rate tests.