Create a Build Script

cargo supports build scripts (you can read the documentation on them here). By creating a build.rs file in our crate directory, cargo will run it before compiling the crate.

build.rs scripts provide a few key features, notably the ability to modify or add linking information during crate compilation. We need our build script to do two things:

1. Compile first-target with the coverage sanitizer enabled
2. Tell cargo (and through it, rustc) to link with the coverage sanitized library when compiling the fuzzer binary