More ASCII85

Huh. I thought marking your MR as Draft would prevent it from auto-closing on merge... alas.

So here's the rest of my ASCII85 rework. I'm reasonably happy with it and ready to move on. 😄 now validates against "ambiguous" encodings, adds some more test files, and provides a somewhat cleaner implementation of act_ascii85 (renamed from act_a85string).

NB: Rationale for renaming the parser ascii85 is that it's the top-level parser for ASCII85, so it gets to claim the name.