New branch for exploring mods to nre's guts
get fix for test error under -DPURIFY
merge trunk: get fix for test error under -DPURIFY
merge trunk
towards an nre stack (instead of linked list)
partial work towards a new tailcall mechanism
 ... tmp
reenable compiled tailcall
more abstraction
Simpler tailcall implementation.

Changed the tailcall implementation to have fewer "moving pieces" and to
better abstract from the details of the NRE "stack" (actually a linked
list). Changed some NRE-stack specific macros.