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

@@ -6,3 +6,15 @@ println("to_str")
 println(a)
 println(['a': 1])
 println(['b': 2 + 2])
+
+println("__index_assign__")
+a['a'] = 1
+println(a)
+a['a'] += 1
+println(a)
+
+foo = () { return 'a' }
+bar = () { return '' }
+
+a[foo() + bar()] += 1
+println(a)