Add Avro-decoder builtin and replacement function

[runarorama]

Overview

This adds a replacement function for decoding Avro binaries, speeding up Avro decoding by 50x or more.

Implementation notes

Essentially this works like the Json encoding/decoding replacement functions, just with more code.

Interesting/controversial decisions

Everything is just in a section of the relevant files, much like the Json replacement functions. Pulling these out into their own modules turned out to be nontrivial without introducing circular dependencies and the like.

Test coverage

Tests are separately implemented in Unison.

[aryairani]

now that stack.yaml has been reverted, just need to fix the build errors.
./scripts/check.sh is a good resource for preparing a happy commit

don't worry about how you resolve the merge conflicts in all-base-hashes.output.md, that file will be regenerated anyway

[aryairani]

Nice

[dolio]

Largely looks good to me. Just saw some cleanup stuff.

I can't really verify the Avro details of course. But from conversations we had, it sounds like you have tests that were catching problems with the implementation.

[dolio]

Looks good now, I think.

[aryairani]

Was this intentional?