hash for IntSet

There is no implementation of `hash(s::IntSet, h::Uint)` for `IntSet`s, and the fallback is `hash(x::ANY, h::Uint)` ([here](https://github.com/JuliaLang/julia/blob/1c5a76d494601464ed7dc6bc393d6848e1954c36/base/hashing.jl#L8)), so `isequal(a::IntSet, b::IntSet)` did not imply `hash(a)==hash(b)` per the [manual](http://docs.julialang.org/en/latest/stdlib/base/?highlight=hash#Base.isequal).