Trivial accessors whitelist

Created a whitelist for trivial accessors. Also cleaned up the trivial accessors cop a bit, because I felt that with this addition the number of checks slapped on to trivial_reader? / trivial_writer? became too much, so split them up in a check to see if it looks like a trivial reader/writer, and a check to see if we allow this method for some reason (whitelist, predicate, initializer, ..)

Fixes #421