Add Rust fuzzing support.

Add a rust_fuzz module which builds a libfuzzer binary that enabes
asan+sancov. This relies on the libfuzzer-sys crate.

Bug: 147140513
Test: Local rust_fuzz example builds, fuzzes with asan+sancov.
Change-Id: I57db3b8d25869791824ccfab768d13b0bb9d42fa
diff --git a/rust/proc_macro.go b/rust/proc_macro.go
index 0c6ec99..f753e7f 100644
--- a/rust/proc_macro.go
+++ b/rust/proc_macro.go
@@ -51,6 +51,8 @@
 		flagExporter: NewFlagExporter(),
 	}
 
+	// Don't sanitize procMacros
+	module.sanitize = nil
 	module.compiler = procMacro
 
 	return module, procMacro