Implement proc macro feature.

[daxpedda]

Implement proc macro alternative to rusty_fork_test!.
Theres a bit of documentation missing and maybe more unit tests could help, I'm willing to do it, but would like some direction.

Fixes #9.

Blocked on #10.

[daxpedda]

Should this even be optional?

[debnil]

@daxpedda: I'd like to use rust-fork to test an async function using tokio, and this PR looks perfect for that.

Are there any plans to merge this? If you need additional community help to finish this, I'm happy to contribute.

[daxpedda]

I will take a look at it, if anybody who his responsible for the project can answer some of my questions above that would be splendid.
The PR is done, my questions above, except documentation, are purely organizational, but otherwise the PR is ready to be merged.

[daxpedda]

So looking through hit again, I realized that this PR is not as clean as I would like it.
It tries to solve two problems at once, that is implement a proc macro, and return a Result. I think this PR would be better served if #10 would be merged first.
#10 is already finished and ready to be merged as far as I am concerned.

[daxpedda]

I cleaned everything up now:

• added documentation
• added more tests
• updated tokio
• cleaned up macro code to rely on Implement returning Result from test functions. #10

This is now only blocked on #10, which again, is ready to be merged as far as I am concerned.

[debnil]

Wow - thanks so much for the fast turnaround and excellent work!

@AltSysrq: this pull request, along with #10, would extend support for async tests. Merging both would be incredibly helpful. Thank you so much for your great work!