inverval_set: optimize subset_of with sequential search

inverval_set: optimize subset_of with sequential search

Optimize subset_of to use sequential search when it
performs better than the lower_bound method, for set
size ratios smaller than 10. This is analogous to
intersection_of behavior since commit 825470fcf919.

The subset_of method can be used in some cases as a
less-expensive alternative to the intersection_of
method, since subset_of can return early if any element
of the smaller set is not contained in the larger set,
and intersection_of has the added burden of storing
the intersecting elements.

Signed-off-by: Zac Medico <zmedico@gmail.com>