Disallow struct literals in ambiguous positions.

Do not identify struct literals by searching for `:`. Instead define a sub-category of expressions which excludes struct literals and re-define `for`, `if`, and other expressions which take an expression followed by a block (or non-terminal which can be replaced by a block) to take this sub-category, instead of all expressions.
Merge remote-tracking branch 'nrc/parse'
Disallowing some struct literals is RFC 25