Work-in-progress on parser state.
Work-in-progress on parser state.

This adds a cached `next` value to `BasicState`. It looks to me like this
*probably* works, except that it runs into a "classic" GC deficiency / bug:
Running out of stack when marking a deeply-nested structure. I'm committing
this now, so I can set it aside and do the necessary GC rework.

danfuzz/samizdat,danfuzz/samizdat,danfuzz/samizdat,danfuzz/samizdat