Enum pattern compilation

[yannham]

Depends on #1816

Follow-up of #1816. Implement missing enum pattern compilation, making matchv2 able to handle full-fledged pattern matching.

Follow-up

• Special case matching on a bunch of enum tags to restore the previous behavior, which used to index directly in a record containing the body of each branch. Honestly, given the size of the matches, it's not clear that this is a huge gain: for now, I would just add the general "make pattern compilation produce more efficient code" to Performance improvement ideas: tracking issue #1484, and only explore this path if pattern matching is shown to be too slow.

[dpulls]

🎉 All dependencies have been resolved !