for decoding closures.  There is now no notion of a current OS
thread, though that needs to come back in the form of a
current Go thread.  As a result, Process now implements Peek
and Poke and maps them to any stopped OS thread, since they
all share the address space anyways.

R=rsc
APPROVED=rsc
DELTA=322  (310 added, 3 deleted, 9 changed)
OCL=34136
CL=34201