Add an ImmutableBitSetPredicate variant for bitsets <= 128 elements

As a means to optimize URLEncoder, add a specialization to ImmutableBitSetPredicate for small sets (<= 128 elements). This was prototyped as an enhancement to inline the bitset into URLEncoder directly in JDK-8317698. This RFE suggests specializing ImmutableBitSetPredicate for a similar win.