Add index assignment and augmented assignment

This allows for syntax like `foo['a'] = 1` and more complex assignments
like `foo.bar()[a() + b()] += 1`

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

tests/map.npp.expect

@@ -2,3 +2,8 @@ to_str
 [:]
 ['a': 1]
 ['b': 4]
+__index_assign__
+['a': 1]
+['a': 2]
+['a': 3]
+