allow definition of pub struct via mock_trait

[hobofan]

This change allows you to define mock structs with pub visibility, which is nice if you want to centrally define a mock struct to be reused in many tests.

It looks like this:

   mock_trait!(
        pub MockStruct,
        somefunction(String) -> String
    );

Sadly the implementation for this involves a lot of duplicate code, but accoring to rust-lang/rust#18317 (comment) and other things I've found, there doesn't seem to be another way to do that.

[DonaldWhyte]

LGTM. Yeah, it's a shame that there is currently no 0-or-1 operator in macro definitions. That would remove the duplicate code. I've added an issue to remove this duplicate when the relevant changes have been pushed to stable Rust. The issue is here:

#7

[DonaldWhyte]

I'll publish a new version of the double crate this evening (UK time).

[DonaldWhyte]

Published version 0.1.4 to Cargo, which should contain this change.