Fix freshening substitutions

[lpw25]

This fixes an issue where freshening substitutions might be applied to module types that had already been freshened. In the case of an identifier collision this produced the wrong result. This bug has probably been around for a long time (maybe since module aliases were added) but we only happened to see an actual collision recently.

[stedolan]

For the record: while this looks like (and is) a straightforward refactoring, the actual bug fix is in Env.scrape_alias: the old code sometimes returned a signature without the freshening substitution applied and sometimes with.

The result of scrape_alias was later freshened, which meant that it could be freshened twice. If the freshened IDs happened to collide with distinct unfreshened identifiers that happened to have the same name then the signature gets corrupted (due to the amount of "happened to" needed to trigger this, the bug was rarely hit). The fix here is to ensure everything is always freshened immediately, using the new Subst.Lazy module to do so efficiently.

[stedolan]

@lpw25: Could you propose this change upstream as well?

Also, the CI failure on principality-and-gadts is almost certainly ocaml/ocaml#10638, so it might be worth porting that patch too.

[poechsel]

Here's a commit fixing the tests:
poechsel@6fba25b

[lpw25]

Submitted upstream here: ocaml/ocaml#10659 stealing some of your comment for the PR description.