The backtrace is now nested, and should be used together with the stack
pointer array to construct the full backtrace for each exception.
We now allocate exception objects in a stack, but their names are still
not allocated. This is fine for exceptions raised in the driver or artiq
code, but we will have to implement allocation for names of exceptions
raised in RPC calls. The compiler should also emit code to store the
exception names once they catch it, to prepare for later reraising.
