PHPLIB-1184: Improve CallbackIterator

[alcaeus]

PHPLIB-1184

#1059 contained some improvements to the CallbackIterator. These have been extracted here:

• Pass element key to the callback
• Define template for types in CallbackIterator
• Declare the expected type for the callback
• Accept any callable as callback instead of only accepting a Closure instance

[jmikola]

I'd propose rewriting all of these tests to use the following matrix:

• Callbacks in various forms that apply the same transformation (*= 2 seems fine)
  • Closure
  • Object method (e.g. static method on this class)
• Iterables
  • ArrayIterator constructed from an indexed array
  • ArrayIterator constructed from an associative array (to show that keys are preserved)
  • IteratorAggregate for both of the above (per @GromNaN's suggestion below)

Using a data provider for iterables and expected arrays would probably be most efficient.

[GromNaN]

@alcaeus I can make the changes if it frees you up.

[GromNaN]

LGTM