Deduplicate mock_trait Macro

[DonaldWhyte]

Currently, the mock_trait has two duplicate definitions. One is for private mocks and the other is for public (pub) mocks.

As shown by the following Rust issue, there is currently no way to define a single macro that has an optional (i.e. 0-or-1) token to be included:

rust-lang/rust#18317

This issue is here so we don't forget to deduplicate this macro when the relevant changes have been made to Rust to allow optional tokens.

Originated from PR: #6

[DonaldWhyte]

I'm closing this issue. #10 has just landed, which mostly removes most of the duplicated code in mock_trait! caused by the pub/non-pub versions.

I don't think there's a reason to wait for issue rust-lang/rust#18317 to be resolved for this, since most of the duplicated has been removed now.