Add fuzzing function for Parse()

[fernape]

This function will receive inputs with a high variability. The fuzzer should help us catch any problems early as we complete the parser.

Run with go test --fuzz=Fuzz

[manadart]

@Aflynn50 Prefer rebasing in-flight branches over merging the mainline back in. It makes for a cleaner commit history.

[niemeyer]

Thanks for fixing it, Alastair.