[`let_chains`] Forbid `let` inside parentheses
Parenthesizes are mostly a no-op in let chains, in other words, they are mostly ignored.
```rust
let opt = Some(Some(1i32));
if (let Some(a) = opt && (let Some(b) = a)) && b == 1 {
println!("`b` is declared inside but used outside");
}
```
As seen above, such behavior can lead to confusion.
A proper fix or nested encapsulation would probably require research, time and a modified MIR graph so in this PR I simply denied any `let` inside parentheses. Non-let stuff are still allowed.
```rust
fn main() {
let fun = || true;
if let true = (true && fun()) && (true) {
println!("Allowed");
}
}
```
It is worth noting that `let ...` is not an expression and the RFC did not mention this specific situation.
cc `@matthewjasper`
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