intbitset: 32-bit len implementation fix

• The optimization introduced in e16df32c3ad48a830f3894b27f2fc0983082a790 was not compatible with 32-bit architectures. This is now fixed. (closes #1132)

• Reduces the number of sets tested in the unit test suite, added in 6511a023ef60bf53a289e13257e8053e939604c6, in order to improve unit test speed, while still checking for 32/64 bit corner cases as in the original commit.