Calling conveys multiple times does not work

[gdamore]

Because of our use of static variables to hold setjmp information, it does not work to call Convey() from different call sites or multiple times. For example, two functions cannot call a 3rd function that embeds a Convey() block.

This is a major limitation. We need a way to understand the context of the caller, and either reset it, or cope. I'm not sure how to solve this at present; probably the static variable approach is just inadequate.