perf: flat doc stream

Docs are one stream of elements, emitted in order: a container records
where it ends, so a concatenation is elements next to each other and
needs no node or list. The printer keeps Prettier's command stack, with
each command a range of the stream, so fits and the rest commands mean
what they mean in Prettier. Breaks propagate as containers close.

The HTML printer emits in order; embedded code is formatted before
anything is emitted, so a hook that declines leaves nothing behind.
Output is the same on all 634 files of the corpora and fixtures.
Pages on one thread: 250 to 207 ms.