Adding ability to mock extern "C" functions

[DavidDeSimone]

Adding ability to mock extern "C" functions defined by the emacs C layer. This is done via a mocking library using proc macros. Adding a basic test as an example.

This is the solution that was discussed in #299 It is using a library that I maintain, which can be found here: https://github.com/DavidDeSimone/mock_derive

[brotzeit]

I don't feel confident enough to review PRs. Hopefully somebody with more experience has the time to take a look at it soon.

[DavidDeSimone]

No problem @brotzeit . As a call out, this crate should only affect the app when running in test configuration. It should have zero affect on symbols/variables/etc when running cargo build or cargo build --release

[Wilfred]

LGTM :)

I'm sure we'll polish this as we expand our test coverage, but it is absolutely brilliant to have everything we need to write unit tests of our low-level code :)