let in go block leaks memory

When a let binding gets used across a blocking operation, the binding leaks out of the let.

The stuff binding gets stuck into the the state-machine state. This persists across the recur, until the binding is next encountered.

(In our case, a few GBs had accumuled by that time.)

Ideally, the fields in the state that represent local variables would be nulled when the variables go out of scope.