Merge 5.2.0minus-5

[liam923]

Merge flambda changes for 5.2.0minus-5. The first two commits run the import script and commit merge conflicts - I recommend skipping them when reviewing.

This PR was difficult for two reasons:

1. Unboxed types caused a lot of conflicts, and broke some Merlin code. For now, the Merlin support for unboxed types is not very good. I added a test file that demonstrates what works and what doesn't. I've internally made a ticket to improve this state of affairs.
2. Due to overwriting, the compiler added "holes". The compiler's notion of holes is for statements like overwrite (3,4) with (_, 5). New nodes are added to the parsetree and typedtree: Pexp_hole and Texp_hole, respectively. This partially conflicts with Merlin's idea of typed holes. Before, Merlin used an extension node to represent holes in the parsetree, and in the typedtree, it also added a Texp_hole constructor. I resolve this by using the new Pexp_hole in place of an extension node (so Pexp_hole represents either type of hole) and renaming Merlin's Texp_hole to Texp_typed_hole (thus, there is now both Texp_hole and Texp_typed_hole in Merlin). I've submitted a PR to upstream Merlin to rename Texp_hole to Texp_typed_hole to mitigate conflicts in the future. (I also renamed Tmod_hole to Tmod_typed_hole for consistency - I included this in the upstream PR as well.)

[liam923]

I'm a bit confused by the CI failure. The same test passes when I run locally, and I haven't yet been able to reproduce. This test has been known to be flaky in the past, so I re-ran it twice, but there's two test cases that have consistently failed across all three runs. I'm tempted to comment out those tests for now so that they don't block rolling Merlin (the justification being that they were already flaky before).

[goldfirere]

Those tests shouldn't be flaky -- they're an output of type-checking, which should be deterministic. That said, I'm not shocked that this has changed as we've evolved the type checked. The tests are looking to see where we allocate a cell that never gets used, which is a bit unusual. I think it's fine to accept the new output. But I wouldn't comment out the tests!

[liam923]

I'm hitting merge without waiting for replies to my replies in order to roll this Merlin version. If more changes are needed, please lmk and I'll make them in a separate PR. (I don't foresee any requested changes being ones that should block rolling)

[goldfirere]

Yes all good. Thanks!