Group cosets: implement custom in, improve iterator

[fingolfin]

Right now in for group cosets resorts to iterating over the coset, which is very inefficient.

We should provide a custom method for this. I would not dispatch to GAP's, but instead implement this "directly": i.e. to test g in Hx, test if g/x in H and similar for g in xH (so that we avoid creating GapObj for those).

---

Speaking of iterators, the iterate implementation for GroupCoset currently directly wraps a GAP coset iterator. But (a) this requires creating a GapObj (with extra overhead for left cosets). And furthermore on the GAP side, the iterator is a wrapper for an enumerator.

Instead, the Julia iterator for a coset C could be based on the group iterator for C.H.

---

Lastly, contrast

Base.IteratorSize(::Type{<:GAPGroup}) = Base.SizeUnknown()
Base.IteratorSize(::Type{PermGroup}) = Base.HasLength()

versus

Base.IteratorSize(::Type{<:GroupCoset}) = Base.SizeUnknown()

I suggest we use something like this:

Base.IteratorSize(::Type{<:GroupCoset{T}}) where T = Base.IteratorSize(T)

And then I assume similar for GroupDoubleCoset.