Fixes #12350.
Parentheses around assignment statements such as
```rust
let mut a = (0);
a = (1);
a += (2);
```
are not necessary and therefore an unnecessary_parens warning is raised when
statements like this occur.
NOTE: In `let` declarations this does not work as intended. Is it possible that they do not count as assignment expressions (`ExprAssign`)? (edit: this is fixed by now)
Furthermore, there are some cases that I fixed in the rest of the code, where parentheses could potentially enhance readability. Compare these lines:
```rust
a = b == c;
a = (b == c);
```
Thus, after having worked on this I'm not entirely sure, whether we should go through with this patch or not. Probably a matter of debate. ;)