syntax: allow stmt/expr macro invocations to be delimited by [].

this is useful for macros like vec! which construct containers

This is a followup to @huonw's earlier change which allowed `{}` to be used as delimiters. I'm acting at the behest of @eddyb on IRC, who also wanted `vec![a, b, c]`. Is this the sort of change which now requires going through the RFC process?