* Complete deprecation of stored zeros; IntSets now only support integers in the range `1:typemax(Int)`
* Complete deprecation of `complement`; removes all support for inverted IntSets
* Refactor internals to rely on a BitVector, allowing the use of highly optimized `map` methods. `IntSet` is now immutable. This significantly improves performance across varying [densities](http://imgur.com/a/uqv8A) and [sizes](http://imgur.com/a/iEgcr). These are compared against a modified Base with deprecation warnings removed for a fairer comparison. Testing code [available here](https://github.com/mbauman/IntSets.jl/tree/b50a7c97abbe9786e33221f723e107e266f31fe4/test).
* Add more tests and organize into testsets.
* Improve hashing; `hash(IntSet([1]))` is now distinct from `hash(IntSet([65]))`

This is a continuation of #10065. Now that complements are fully removed, making IntSet immutable solves the performance issue. I am keeping the name the same within this PR as it vastly simplifies comparisons between the two implementations; the name can later be changed to `IndexSet` if still desired. The naming story is now a bit more complicated since we support offset indices, but a future change could perhaps allow wrapping any `AbstractVector{Bool}` and base the supported `Int`s on those indices. Very few methods depend upon BitArray internals.