generate test coverage

[seanmonstar]

it'd be awesome if there was a Rust code coverage tool already. I think there's work around being compatible with kcov, but something that modifies the source and inserts counters would be awesomer.

[reem]

Relevant: rust-lang/rust#690

[reem]

I am under the impression there is a relatively easy way to do this nowadays vs. the ancient times, so tagging easy.