fixes #606: allow unquoted reserved words in more positions

My first significant pull request :smile:. Fixes #606. Allows reserved words to be used in record literal keys, property updates, property accessors, destructurings, and record type signatures.
Not sure it makes much of a difference either way, but I think we use applicative style in the parser most of the time at the moment whenever it's not ridiculous. I could be wrong though, just going by memory!