Old code was using recursion to traverse object graph.
New code uses an explicit stack, cutting the per-pointer
footprint to two words during the recursion and avoiding
the standard allocator and stack splitting code.
in test/garbage:
Reduces parser runtime by 2-3%
Reduces Peano runtime by 40%
Increases tree runtime by 4-5%
R=r
CC=golang-dev
https://golang.org/cl/2150042
updated thread.c to provide destroylock, which seems to be
required to link.
updated README with different virtualization programs.
R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/1746047
