Expressions other than calls

[shashi]

It could be cool to have rules like:

@rule let ~assignments1
    let ~assignments2
       ~body
    end
end => let vcat(~assignments1, ~assignments2)...
     ~body
end

Where let maps to SymbolicUtils.Code.Let(assignments, body), etc.

Can be useful in doing various things inside build_function.

cc @0x0f0f0f