handle numeric fields in struct syntax

[tjhance]

Did you know you can do:

enum Foo {
    Bar(u32, u32),
    Qux,
}

fn test() {
    let b = Foo::Bar { 1: 30, 0: 20 };
    assert(b.get_Bar_0() == 20);
    assert(b.get_Bar_1() == 30);
}

Well, I didn't! I only realized it because rustc's internal de-sugaring of '?' generates code that uses this form. Then I checked, and it turns out users can write it this way too. This PR handles the numeric identifiers correctly (previously, Verus would panic because it didn't translate the to VIR correctly).

VIR previously had this note for struct-syntax patterns and struct-syntax constructors:

/// For tuple-style variants, the field initializers appear in order and are named "_0", "_1", etc.
/// For struct-style variants, the field initializers may appear in any order.

The "appear in order" part is no longer true as of this PR. It seems nobody was relying on it, anyway, but it would be good for someone other than me to double-check.

[utaal-b]

Will this store the name in key_to_name even for numeric fields for which we are not going to use the rename_count in lifetime_emit? Does that matter?

[tjhance]

Yes

[utaal-b]

Why is the distinction in lifetime_emit necessary?

[tjhance]

I guess it's not necessary, now that I think about it. I guess the original reason I did it was because I didn't think it made sense to use IdKind::Local for field names.