laziness of conditionals

[msaaltink]

The reference evaluator and REPL evaluator disagree again:

Cryptol> if error "boo" then () else ()

Run-time error: boo
Cryptol> :eval if error "boo" then () else ()
()

Similarly with infinite recursion:

Cryptol> if b then () else () where b = b

<<loop>> while evaluating (at <interactive>:1:28--1:29, <interactive>::b)
Cryptol> :eval if b then () else () where b = b
()

[robdockins]

This seems like essentially the same problem as #422. Basically, we should only demand the value of the conditional test if the caller is going to demand at least one bit of the output... and there are no bits of output at the unit type.

[brianhuffman]

Duplicate of #422.