custom mutator rust support (#752)

* custom mutator rust support

* clarify how to view documentation for rust mutators

* remove `FuzzResult` hack and clarify lifetimes of CustomMutator::fuzz

* rename TErr associated tyep to Error to be more idiomatic

* fix warnings

* add example for fallible custom mutator

* make Fallible Custom Mutator the default and implement it's handle_err method by default

* rename CustomMutator::handle_err to handle_error

* add example mutator using lain

custom_mutators/rust/example/src/lib.rs

@@ -0,0 +1,50 @@
+#![allow(unused_variables)]
+
+use custom_mutator::{export_mutator, CustomMutator};
+use std::os::raw::c_uint;
+
+struct ExampleMutator;
+
+impl CustomMutator for ExampleMutator {
+    type Error = ();
+
+    fn init(seed: c_uint) -> Result<Self, ()> {
+        Ok(Self)
+    }
+
+    fn fuzz<'b, 's: 'b>(
+        &'s mut self,
+        buffer: &'b mut [u8],
+        add_buff: Option<&[u8]>,
+        max_size: usize,
+    ) -> Result<Option<&'b [u8]>, ()> {
+        buffer.reverse();
+        Ok(Some(buffer))
+    }
+}
+
+struct OwnBufferExampleMutator {
+    own_buffer: Vec<u8>,
+}
+
+impl CustomMutator for OwnBufferExampleMutator {
+    type Error = ();
+
+    fn init(seed: c_uint) -> Result<Self, ()> {
+        Ok(Self {
+            own_buffer: Vec::new(),
+        })
+    }
+
+    fn fuzz<'b, 's: 'b>(
+        &'s mut self,
+        buffer: &'b mut [u8],
+        add_buff: Option<&[u8]>,
+        max_size: usize,
+    ) -> Result<Option<&'b [u8]>, ()> {
+        self.own_buffer.reverse();
+        Ok(Some(self.own_buffer.as_slice()))
+    }
+}
+
+export_mutator!(ExampleMutator);