Broken behaviour when an LC term is called, inside a callback that is passed to an LC term.

The title is rather confusing. Specifically, if an LC abstraction was to be passed to an external JS function, it was wrongly using a reference to the current stack (where it should not have had a stack at all). This meant that when called, it would produce side effects in the current stack, breaking things.

Patch coming soon, issue is just for documentation.