Make the `DependencyInner::specified_req` field a bool.

Make the `DependencyInner::specified_req` field a bool.

This field is hardly used. I could almost get rid if it completely, but
then the dependency verifier would have to abort on, say, no version req
specified or the any req. Crates.io indeed doesn't want such wildcards, but
other registries some day may have a different policy.

Making it a mere bool---since all the information it contained is in the
`req` field anyways---was the next best thing.