Merge #2592

2592: Add std::ops::Index support for infering r=edwin0cheng a=edwin0cheng

see also #2534

Seem like this can't fix #2534 for this case:

```rust
fn foo3(bar: [usize; 2]) {
    let baz = bar[1];   // <--- baz is still unknown ?
    println!("{}", baz);
}
```

Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>

diff --cc crates/ra_hir_def/src/path.rs
Simple merge

diff --cc crates/ra_hir_ty/src/infer/expr.rs
Simple merge