Split crate into `libfuzzer` and `libfuzzer-sys`

[fitzgen]

libfuzzer-sys will just build and link libfuzzer.a.

libfuzzer will depend on libfuzzer-sys and additionally export our nice fuzz_target! macro.

I started poking at this over in https://github.com/fitzgen/libfuzzer/tree/split-libfuzzer-and-libfuzzer-sys but quickly ran into linker issues. Don't have the time for that today, so I'm filing this issue for posterity.