internal: Generally improve make::match_arm

`make::match_arm` should take a single `ast::Pat`, and callers can handle creating an `ast::OrPat` if need be. It should also take a proper `ast::MatchGuard`, instead of making one itself.

crates/syntax/src/syntax_editor.rs

@@ -335,7 +335,7 @@ mod tests {
     #[test]
     fn basic_usage() {
         let root = make::match_arm(
-            [make::wildcard_pat().into()],
+            make::wildcard_pat().into(),
             None,
             make::expr_tuple([
                 make::expr_bin_op(