Rework ast::BinOpKind::to_string and ast::UnOp::to_string.

- Rename them both `as_str`, which is the typical name for a function
  that returns a `&str`. (`to_string` is appropriate for functions
  returning `String` or maybe `Cow<'a, str>`.)
- Change `UnOp::as_str` from an associated function (weird!) to a
  method.
- Avoid needless `self` dereferences.

compiler/rustc_parse/src/parser/stmt.rs

@@ -387,7 +387,7 @@ impl<'a> Parser<'a> {
             if op.node.lazy() {
                 self.sess.emit_err(errors::InvalidExpressionInLetElse {
                     span: init.span,
-                    operator: op.node.to_string(),
+                    operator: op.node.as_str(),
                     sugg: errors::WrapExpressionInParentheses {
                         left: init.span.shrink_to_lo(),
                         right: init.span.shrink_to_hi(),