Alek Ratzloff
365bee0554
Add support for +=, -=, *=, and /= operators. This is basically just
syntactic sugar, but it's still nice to have
a += 1
compiles to the equivalent of
a = a + 1
with all the same implications of scoping rules.
Signed-off-by: Alek Ratzloff <alekratz@gmail.com>