Guards

If we want to capture part of a single pattern, we can do:

```rust
if let Some(foo_contents) = foo {
// foo_contents binding is valid here
}

// foo_contents binding is not valid here
```

In some cases this forces an extra layer of nesting. It would be nice to be able to do something like the following:

```rust
let Some(foo_contents) = foo else {
// foo_contents binding is not valid here
}

// foo_contents binding is valid here
```

Start by reviewing the proposed Rust syntax and its binding-scope examples in the issue. Determine the language-design and implementation areas affected, then define the accepted behavior and supporting tests before considering the work done.

Written by the indexing model from the issue text.