This has to re-introduce the `sink` pattern, because doing this purely
with iterators is awkward :( Maaaybe the event vector was a false start?
But, anyway, I like the current factoring more -- it sort-of obvious
that we do want to keep ws-attachment business in the parser, and that
we also don't want that to depend on the particular tree structure. I
think `shortcuts` module achieves that.
The general theme of this is to make parser a better independent
library.
The specific thing we do here is replacing callback based TreeSink with
a data structure. That is, rather than calling user-provided tree
construction methods, the parser now spits out a very bare-bones tree,
effectively a log of a DFS traversal.
This makes the parser usable without any *specifc* tree sink, and allows
us to, eg, move tests into this crate.
Now, it's also true that this is a distinction without a difference, as
the old and the new interface are equivalent in expressiveness. Still,
this new thing seems somewhat simpler. But yeah, I admit I don't have a
suuper strong motivation here, just a hunch that this is better.
FragmentKind played two roles:
* entry point to the parser
* syntactic category of a macro call
These are different use-cases, and warrant different types. For example,
macro can't expand to visibility, but we have such fragment today.
This PR introduces `ExpandsTo` enum to separate this two use-cases.
I suspect we might further split `FragmentKind` into `$x:specifier` enum
specific to MBE, and a general parser entry point, but that's for
another PR!
