Move the cached sparse sets to the Config.  I tested make.bash with
pre-allocating sets of size 150 and not caching very small sets, but the
difference between this implementation (no min size, no preallocation)
and a min size with preallocation was fairly negligible:

Number of sparse sets allocated:
Cached in Config w/none preallocated no min size    3684 *this CL*
Cached in Config w/three preallocated no min size   3370
Cached in Config w/three preallocated min size=150  3370
Cached in Config w/none preallocated min size=150  15947
Cached in Func,  w/no min                          96996 *previous code*

Change-Id: I7f9de8a7cae192648a7413bfb18a6690fad34375
Reviewed-on: https://go-review.googlesource.com/19152Reviewed-by: Keith Randall <khr@golang.org>