Allow for underscores in match arms

[klinvill]

It's convenient to use underscores in match arms to both concisely capture the remaining arms, as well as to avoid naming variables that won't be used. For example, the following match statement should be supported for options:

#[define]
fn is_some<T>(v: Option<T>) -> bool {
    match v {
        Option::Some(_) => true,
        _ => false,
}

Underscores are used both in place of variables that don't matter (Some(_)) as well as to capture remaining match arms (_ => false).