Matches: guards are dropped!

[W95Psp]

The following example:

fn f(x: u8) -> bool {
    match x {
        _ if x > 20 => true,
        _ => false,
    }
}

Open this code snippet in the playground

Is translated without any guards!

Action items:

• add guards in arms in the AST, feature-gate them
• add a rejection phase and reject them for most backends (at least for now)
• investigate how mutation behaves there

[W95Psp]

Closed by #821