Range, arithmetic reducers, and effectors

Added

• Effectors
  • iter.forEach() (like Array.prototype.forEach)
  • iter.lazyObserver() for observing each yielded value lazily.
• Generators
  • iter.create.range() for creating an iterable over a range of numbers.
• Reducers
  • iter.sum() for getting the sum of an iterable.
  • iter.product() for getting the product of an iterable.
  • iter.norm() for getting the vector norm of an iterable.