Support conditions that can consume iterators/generators.

[pschanely]

Right now, CrossHair doesn't behave well if you consume an iterator or generator inside a condition:

• Doing so on a precondition leaves the input empty for the function body
• Doing so in a post condition appears to work, but will break when checking the condition of another function that calls it.

Possibly, we could use the heap checkpointing logic that we use to implement __old__ to resolve this.