L1_doPushLastLocal

Push a local variable -- not its value, but the variable itself. This should be the last use of the variable, so erase it from the continuation at the same time.

Clearing the variable keeps the variable's reference count from changing, so it may stay mutable if it was before.

If an argument is specified then push the value, since there is no actual variable to operate on. Clear the slot of the continuation reserved for the argument. Constants are treated like ordinary local variables, except that they can not be assigned after their definition, nor can a reference to the constant be taken.