Fix #573: Tuple methods skipping sequential tail

Fixes #573. I recomment viewing 9d98acb in [ignore whitespace mode](https://github.com/ceylon/ceylon.language/commit/9d98acb3e5236bc244cb54a5b11789506e85f605?w=1), it makes the changes clearer (hides the indentation changes).

All `ceylon.language` tests pass. I get some `ceylon-compiler` failures, but I don’t think they’re related (I haven’t tested without the changes, but I assume the failures will be the same).