Reuse existing target directory

[thomaseizinger]

Previously, each synthetic project got its own target directory. That results in a lot of recompilation. Because the entire workspace of cargo semver-checks is within the original projects target directory, we can simply point the cargo doc process to that one.

This allows us to reuse a lot of compilation artifacts up to the point where an invocation of cargo semver-checks only takes a couple of seconds if the user already has a debug build locally or a cache in CI.

Originally, I thought that walking up two directories is a bit of a hack but it is actually extremely stable in my testing. It works with custom target dirs (CARGO_TARGET_DIR) because our entire workspace is within the parent project's target dir.

Thoughts?

Resolves #401.

[obi1kenobi]

Thanks for the ideas and PR!

Let's keep the discussion here focused on the specific implementation of the idea, and discuss the idea itself in #401. I'll switch to the issue so we can sort that out first.