Add array/slice
Use array/slice instead of Array.prototype.slice
add `function/identity`. change `function/makeIterator_` behavior. closes #147

also add test to `array/sortBy` without a callback (make sure it uses identity
function).

this might break some code that expects the internal `makeIterator_` method to
return `null` or `undefined`... but since this is an internal method I don't
think it's a big deal (our code will work fine, all tests still passes).

one downside of the new `makeIterator_` behavior is that it will not throw
errors in cases where user forgot to pass the callback, which might make it
harder to spot why some code is giving the wrong output, but I think it is an
improvement overral since it will increase flexibility and this behavior might
be shared among multiple methods (identity is a good default behavior for
`array/map` as well).