Ergonomic Improvements for Rust Custom Mutators (#772)

* allow returning of str instead of CStr in rust custom mutator

* use OsStr instead of CStr for file paths

* fix cfg and compiler errors

* custom mutator default descriptions

* fix usage of afl_internal feature flag

* fix example mutator cfg

* fix lain mutator

* Revert "fix lain mutator"

This reverts commit adf7001808.

* actually fix lain mutator

* resolve question around utf-8 null bytes

* change from OsStr to Path to be even more ergonomic

* add rust custom mutator ci

* fix github action

* again

* separate compilation check

custom_mutators/rust/example/src/example_mutator.rs

@@ -1,3 +1,4 @@
+#![cfg(unix)]
 #![allow(unused_variables)]
 
 use custom_mutator::{export_mutator, CustomMutator};