Implement grouped types nicely

After @gavinking’s ceylon/ceylon-spec@8990131, we can now just skip that
whole “optional tokens” shenanigans. (Thanks Gavin!)

(I haven’t removed optional tokens from FormattingWriter yet, but as
they’re no longer used now, I’ll most likely do that in another commit
soon.)

Closes #58.
Fix really stupid bug in DefaultLineBreaks

FormattingWriter.LineBreak refers to the queue element type.
LineBreak used to refer to that, too, but the behavior of the import on
top changed (ceylon/ceylon-spec#989), and now it refers to the generated
LineBreak (from options, lf/crlf/os).

This took decidedly too long to find, it’s a really stupid mistake :-(
Remove unneeded imports and misleading comments

ceylon/ceylon-spec#989 was not a bug, we don’t actually want to revert
abf47c2.
Remove optional tokens from FormattingWriter

They were an ugly wart in the interface that, after #58, we no longer
need :)