Merge #2164

2164: Silence some warnings r=matklad a=lnicola

```
warning: unnecessary parentheses around type
   --> crates/ra_tt/src/buffer.rs:114:32
    |
114 |     pub fn end(self) -> Option<(&'a Subtree)> {
    |                                ^^^^^^^^^^^^^ help: remove these parentheses
    |
    = note: `#[warn(unused_parens)]` on by default

warning: unnecessary parentheses around type
   --> crates/ra_tt/src/buffer.rs:130:30
    |
130 |     fn entry(self) -> Option<(&'a Entry<'a>)> {
    |                              ^^^^^^^^^^^^^^^ help: remove these parentheses
```

Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>