The libunwind library version installed includes an rc number. The
latest released version of the `unwind-sys` crate doesn't handle this
properly, so rely on the latest commit in the upstream repository.
This is the start of the organization of types that are in their own
crates in order to break dependency cycles between other crates. The
idea here is that putting these packages into their own directory is the
first step toward cleaning them up. They have grown organically and it
is difficult to explain to new folks where to put new shared types. Many
of these crates contain more than traits or don't contain traits at all.
Notably, `script_traits` isn't touched because it is vendored from
Gecko. Eventually this will move to `third_party`.
This will ultimately make it simpler to update crate dependencies and
reduce duplicate when specifying requirements. Generally, this change
does not touch dependencies that are only used by a single crate. We
could consider moving them to workspace dependencies in the future.
