Make fuzzer easier to use

[Ollrogge]

Main idea is to remove the need for people to create their own harness_fuzz.go and harness_test.go and having to deal with compiling the fuzzer and the coverage binary independently. Approaches work the as follows:

For fuzzing
Copy harness_fuzz.go into the harness directory passed by the user, compile fuzzer, remove harness_fuzz.go. This is implemented in build.rs.

Obtaining coverage
Copy harness_test.go into harness directory, run the test passing in the output directory of the fuzzer, remove harness_test.go from directory. This has been added as an additional command to main.rs.

The coverage file is not optimal since we can not grep for the specific project the user wants to fuzz and therefore it shows the coverage in all dependencies, so also the Go standard libraries.