Full performance review

[iamed2]

We should compare performance to other implementations and consider whether any design changes could improve performance. Alternate implementations which lose out could be made into benchmark tests.

• [ ] Review use of length and iteratorsize
• [ ] Compare performance against Python itertools
• [ ] Compare performance against implementations in Base or other Julia packages
  • [ ] Base
  • [ ] Combinatorics.jl
  • [ ] Lazy.jl
  • [ ] LazySequences.jl