introduce [*] for unknown length pointers

See #770

Currently it does not have any different behavior than `*`
but it is now recommended to use `[*]` for unknown length
pointers to be future-proof.

Instead of [ * ] being separate tokens as the proposal
suggested, this commit implements `[*]` as a single token.
f06bce5ddaAndrew Kelley committed on 6/2/2018, 8:04:23 AMparent7b386ea
9 files changedLine totals unavailable