Parse guard patterns

[dtolnay]

• RFC: Guard Patterns rust-lang/rfcs#3637
• Tracking issue: Tracking issue for RFC 3637: Guard patterns rust-lang/rust#129967
• Feature gate: #![feature(guard_patterns)]

match user.subscription_plan() {
    (Plan::Regular if user.credit() >= 100) | (Plan::Premium if user.credit() >= 80) => {...}
}