intsets.Sparse is a sparse bit vector. It uses space proportional
to the number of elements, not the maximum element (as is the case for a dense bit vector).
A forthcoming CL will make use of it in go/pointer, where it reduces
solve time by 78%. A similar representation is used for Andersen's
analysis in gcc and LLVM.
+ Tests.
LGTM=sameer, crawshaw, gri
R=gri
CC=crawshaw, golang-codereviews, sameer
https://golang.org/cl/10837043
