refactor rustc-hash integration

[llogiq]

I found that rustc-hash is used in multiple compiler crates. Also some types use FxBuildHasher whereas others use BuildHasherDefault<FxHasher> (both do the same thing).

In order to simplify future hashing experiments, I changed every location to use rustc_data_structures::fx::* types instead, and also removed the BuildHasherDefault variant. This will simplify future experiments with hashing (for example trying out a hasher that doesn't implement Default for whatever reason).

[rustbot]

Some changes occurred in exhaustiveness checking

cc @Nadrieril

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[rustbot]

r? @eholk

rustbot has assigned @eholk.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[llogiq]

Ok, I removed the rustc_data_structures. dependency, leaving only the change of BuildHasherDefault<FxHasher> to FxBuildHasher (because that's one less type to import, and leaves us the option of changing the algorithm in the future to something that doesn't impl Default).

[llogiq]

Ok, this compiles now and passes all tests.