Improve speed

[szabgab]

In the open source project I am working on cargo test takes 4 minutes while cargo tarpaulin --ignore-tests -o Html -o Lcov --timeout 240 takes 17 min. Using --engine llvm reduced that to 14 minutes and out of that 14 min the compilation takes 4.

It is still 2.5 times longer than running the tests alone. It would be nice if the speed could be further improved.

If you are interested this is the first run with llvm