Currently when type-safe return values are used, one can't use
`InOrder` neither `After` with them as they expect
`*gomock.Call` as parameters. This changes the type these functions
expect to be an interface which implements the `GetCall() *Call` method,
which returns the embedded `*gomock.Call` type for generated mock types.

Using this approach helps to have compile-time error/warnings instead of
runtime errors with a reflection based solution.

Fixes (#70)

Add a new exported interface `CallWrapper` which allow users to use
`InOrder` and `After` with generated type-safe mock types.