runtime: use manual stack for garbage collection

runtime: use manual stack for garbage collection

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