Lua coroutines can yield across non-coroutine function boundaries,
meaning that we don't need to wrap each helper function in a coroutine
and resume it within `_parse()`. If we just have them yield when
appropriate, this will be caught by the top level `_parse()` coroutine,
and resuming the `_parse()` will resume from the position in the helper
function where we yielded last.
0c9c140f91
