Define a fuzz target for the parser

cranelift/isle/fuzz/fuzz_targets/parse.rs

@@ -0,0 +1,9 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|s: &str| {
+    let lexer = isle::lexer::Lexer::from_str(s, "fuzz-input.isle");
+    let mut parser = isle::parser::Parser::new(lexer);
+    let _ = parser.parse_defs();
+});