Instead, split the underlying storage in half and
free just half of it.

Shrinking without copying lets us reclaim storage used
by a previously profligate Go routine that has now blocked
inside some C code.

To shrink in place, we need all stacks to be a power of 2 in size.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/69580044