Bind an expression to the "it" variable _before_ evaluating it

[robdockins]

so we can cache the value via it's thunk. This prevents reevaluating
the term when the "it" variable is referenced later.

Fixes #881

[brianhuffman]

Looks very straightforward to me.

Do you think a regression test would make sense for this? Maybe something with trace would work, or do you think that would be too fiddly?

[robdockins]

Humm, yeah we could maybe do something with trace. I'll see if that makes sense.

[brianhuffman]

That regression test is the same as what I was thinking of. The behavior in the current master is kind of weird:

Cryptol> traceVal "doing some work" (42 : Integer)
doing some work 42
42
Cryptol> it
doing some work 42
42
Cryptol> it
42
Cryptol> it
42

[robdockins]

Yeah, it demonstrates that we already had the necessary mechanics in place. We basically do a magic let binding to it, which thunks it. That means after the first evaluation of it, the interpreter doesn't evaluate it again. We just needed to do things in the other order!