Use stable rustfmt

[djmitche]

This uses the rustfmt from the stable toolchain, instead of nightly. At the moment, nightly's rustfmt is nonfunctional, but in general we don't need bleeding-edge formatting (and in fact, the more often formatting changes, the more often we'll get spurious failures on PRs, as we are today).

Fixes #2165.

[djmitche]

Once this is merged, I think we can rebase a bunch of other open PRs and land them.

[mgeisler]

in general we don't need bleeding-edge formatting (and in fact, the more often formatting changes, the more often we'll get spurious failures on PRs, as we are today).

This is not 100% true: we use unstable features in our rustfmt.toml file:

# Please use a nightly rustfmt for these settings.
imports_granularity = "module"
wrap_comments = true

Luckily, both of these settings are no-ops with a stable rustfmt, as long as people wrap things nicely by hand 😄

We could instead pin the nightly version used in CI: you're right that we don't want the "latest and greatest" when it comes to formatting, we just want a consistent result and some useful features as well.