Mocking in Rust

[peter-lyons-kehl]

Thank you for blessed-rs Nico.

Suggest adding an entry on Mockall, with 3: links:

1. Comparison to other ways of mocking (in Rust) https://asomers.github.io/mock_shootout,
2. the crate itself: mockall https://docs.rs/mockall/latest/mockall, and
3. its sister crate mockall_double https://docs.rs/mockall_double/latest/mockall_double/index.html.

Suggest adding a new section "Mocking". Of course, up to you where to put it/what to call it. But, I don't think it fits under current Tooling > Testing, since it's not really a (shell) tool.

(I may have one more, separate, crate to add.)