Add a top-level parse function

And make `parse_defs` take `self` by ownership. This avoids a couple `Vec`
clones.

cranelift/isle/fuzz/fuzz_targets/parse.rs

@@ -9,9 +9,7 @@ fuzz_target!(|s: &str| {
     log::debug!("lexer = {:?}", lexer);
 
     if let Ok(lexer) = lexer {
-        let mut parser = isle::parser::Parser::new(lexer);
-
-        let defs = parser.parse_defs();
+        let defs = isle::parser::parse(lexer);
         log::debug!("defs = {:?}", defs);
     }
 });