Use to Cargo's experimental lockfile format

This commit changes the lock file format of this repository to an
experimental format that isn't rolled out by default in Cargo but is
intended to eventually become the default. The new format moves
information around and compresses the lock file a bit. The intention of
the new format is to reduce the amount of git merge conflicts that
happen in a repository, with rust-lang/rust being a prime candidate for
testing this.

The new format wille ventually become the default but for now it is
off-by-default in Cargo, but Cargo will preserve the format if it sees
it. Since we always build with a beta version of Cargo for the
rust-lang/rust repository it should be safe to go ahead and change the
lock file format here and everyone building this repository will
automatically pick it up.

It's intended that we'll evaluate this lock file format in the
rust-lang/rust repository to see if it reduces the number of perceived
merge conflicts for changes that touch the lock file. This will in turn
help inform the development of the feature in Cargo and whether we
choose to stabilize this and turn it on by default.

Note that this commit does not actually change the contents of the lock
file in terms of a resolution graph, it simply reencodes the lock file
with a new format.