Neat 0.5.3: Prelude to Breakage

• Assignment to index now works through alias:
  alias x = array[0]; x = 5;
• Allow class fields to be declared mut.
  This as yet does nothing; it's just parser prep.

Obviously the big upcoming change in 0.6 here is that class fields will no longer be mutable by default. This will require a lot of rewriting; all I can say in excuse is that it is absolutely necessary for performance reasons. The upside of requiring mut on class fields is that class fields that are not mutable will no longer require a ref increase when accessing the field on a lexical-scoped expression. This should give a moderate but solid speedup going forward.