Eliminated SmallIntSet

[sfackler]

SmallIntSet is equivalent to BitvSet but with 64 times the memory
overhead. There's no reason for it to exist.

SmallIntSet's overhead should really only be 8 times, but for some
reason, sys::size_of::<Option<()>>() == 8, not 1.

[huonw]

The reason that Option<()> is 8 bytes is because the discriminant is always word size, rather than shrinking to be the minimum possible (#1647).

[thestinger]

I don't think these are really equivalent. If you add 1, 2 and 3 to SmallIntSet, it will contain them as 3 distinct elements. If you only insert 1 and 2, it will not contain 3. I might be misunderstanding what BitvSet is, but I thought it was a set of bits.

[huonw]

@thestinger nth bit set == n in the BitvSet. (At least, that's the current semantics.)

[thestinger]

Ah.

[huonw]

@sfackler needs a rebase.

[sfackler]

@huonw Done.