(PUP-2972) Remove += and -=

Previously the future parser supported += and -=. += was very similar to
the same operator that is in the current parser, but with a few
differences to make it consistent with the rest of the
language. The += operator no longer performed string concatenation and
instead acted like $a = $a + $b. The -= operator was added to complete
the set of operators available.

However, many people have been confused by these operators (even as they
exist in the current parser):

  * It looks like mutation, but the language does not have mutation
  * It looks like += from other languages, but doesn't behave like them
  * Shadowing a variable is something that needs to be explicit,
    otherwise it becomes confusing.

This removes += and -= from the future parser. They are still parsed,
but the validation step will issue errors if they are encountered.