Rearrange mwapi/mwapi_responses dependency

We're getting close to mwapi 1.0, but the mwapi_responses crate isn't anywhere close to ready yet. So instead of having mwapi depend upon mwapi_responses, let's invert the dependency and have mwapi_responses optionally depend upon mwapi. The calling syntax is roughly the same so it shouldn't be too difficult to migrate.

The feature is named mwapi-05 for now, to correspond with version 0.5, but it'll eventually be named mwapi-1.