Investigate use of Pyre

[trickeydan]

Kajetan has rightly pointed out that mypy is pretty awful.

Let us be enlighted by Ocaml

[trickeydan]

pyre --source-directory j5 --verbose check
 ƛ Setting up a `.pyre_configuration` with `pyre init` may reduce overhead.
 ƛ Processed 30 of 57 sources [676.6s]

It's been stuck like this eating 100% of my CPU for 10 minutes.

[trickeydan]

I've tried this again with the same result.

I'm going to call this a won't fix.

[kierdavis]

I get different output, but it still sits there pegging 100% CPU for multiple minutes.

Even if it does eventually finish, I'd deem this a major workflow breaker.

[trickeydan]

I'm up for reopening this issue in, say 6 months, to see if it starts working.