Auto merge of #2863 - QuiltOS:specified-req, r=alexcrichton

Auto merge of #2863 - QuiltOS:specified-req, r=alexcrichton

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

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 wildcards, but other registries some day may have a different policy.

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