Proposal: allow switch on packed struct

[mlugg]

This proposal is a simple extension of #21678. The following code should compile:

const S = packed struct {
    a: u1,
    b: u1,
    fn foo(s: S) bool {
        return switch (s) {
            .{ .a = 0, .b = 1 }, .{ .a = 1, .b = 0 } => true,
            else => false,
        };
    }
};

test "switch on packed struct "{
    try expect(S.foo(.{ .a = 1, .b = 0 }));
    try expect(!comptime S.foo(.{ .a = 0, .b = 0 }));
}

const expect = @import("std").testing.expect;

This will require re-introducing result types for switch items, which were removed way back in 85e94fe. They were removed to prepare for #2473, but that proposal has since been un-accepted.