check lambda form body structure more strictly

[porcuquine]

As noted in a comment on #183, we currently allow lambda forms that don't have exactly one body element. It would be better if this led to an error.

Examples:

Lurk REPL welcomes you.
> (lambda (x))
[1 iterations] => <FUNCTION (X) )>
> (lambda (x) 1)
[1 iterations] => <FUNCTION (X) 1)>
> (lambda (x) 1 2)
[1 iterations] => <FUNCTION (X) 1 2)>
> ((lambda (x) 1 2) 9)
[4 iterations] => 1

Since we want to avoid extra destructuring (car_cdr()), which incurs hashing costs in the circuit, the check should not come at the time of creating the function, but later (probably when handling Continuation::Call2 in apply_continuation().), when the body is being inspected.

[weissjeffm]

@porcuquine I tried implementing it the way you specified, but I find it strange that you are allowed to create a lambda with an invalid body, but you can't call it - was that your intention?

Lurk REPL welcomes you.
> (lambda () 1 2 3)
[1 iterations] => <FUNCTION () 1>
> ((lambda () 1 2 3))
Evaluation error: Function body must be only one element
> 

[porcuquine]

Having completed the cons hashing optimizations, I now suspect we can afford to perform the destructuring earlier — so you can go ahead and change it as you suggest. We'll want to ensure that works without adding a hash to the circuit though.