Simplifying control flow operators

[gancherj]

Currently, we have code like:

    if eq(length(M), |nonce|) then
        if eq(length(A), |nonce|) then
            if eq(length(B), |nonce|) then
                case x1 
                | mA2_enc _ => ()
                | mA1_enc x1' => 
                    (case dec(kA, x1')
                        | None => ()
                        | Some resA =>
                            unpack j1, resA' = resA in
                            unpack j2, resA'' = resA' in
                            (case resA''
                                | mA1 msg1A => ...
                                | mA2 _ => ...))

Aside from the nested equality checks, we also want better syntax for nested case expressions, and nested unpacks.
Perhaps something like:

(mA1_enc x1') =? x1 in 
(Some ResA) =? dec(kA, x1') in
...

where the semantics of =? is to return () if the pattern match fails. (Or, perhaps if we return an option, to return None).
A Rust-style if let operation would work well too.