Optimize lock file format for git merge conflicts

This commit is an attempt to refine Cargo's lock file format to generate
less git merge conflicts for lock file updates as well as make it easier
to manage lock file updates. The new lock file format has a few major changes:

* The `[metadata]` table is no longer used to track checksums. The
  packages themselves now list `checksum` fields directly.

* The entries in the `dependencies` key no longer unconditionally
  mention the version/source of the dependency. When unambiguous only
  the name or only the name/version are mentioned.

As mentioned before the goal here is to reduce git merge conflict
likelihood between two cargo updates to a lock file. By not using
`[metadata]` the updates to package checksums should only happen on the
package itself, not in a shared metadata table where it's easy to
conflict with other updates. Additionally by making `dependencies`
entries shorter it's hoped that updating a crate will only either add
entries to a lock file or update lines related to just that package.
It's theorized that the amount of updates necessary to a lock file are
far less than today where the version has to be updated in many
locations.

As with all lock file format changes this new format is not turned on by
default. Support is added so Cargo will preserve it if it sees it (and
tests are added too), and then some time down the road we can flip the
switch and turn this on by default.

Author: alexcrichton