allow binary-only dependencies

[vks]

Binaries usually have a lot of dependencies libraries don't care for. If you want to have different dependencies, you currently cannot build the library and the binary using the same Cargo.toml, resulting in needless duplication of the Cargo.toml.

[alexcrichton]

Thanks for the report! I think this is a dupe of #1982, however, so closing in favor of that.