E0308 should suggest .into() if available

[estebank]

Given:

fn main() {
    let x: A = B;
}

struct A;
struct B;

impl From<B> for A {
    fn from(_: B) -> Self { A }
}

the current output is

error[E0308]: mismatched types
 --> src/main.rs:2:16
  |
2 |     let x: A = B;
  |            -   ^ expected struct `A`, found struct `B`
  |            |
  |            expected due to this

but we should probe for possible Into and From impls and suggest them if reasonable.