Improve performance of iter library ( core::iter )

[catamorphism]

As per FIXMEs:

• implement a riterable interface to support reversed()
• make sure min and max are optimized to use moves for the accumulator value

[brson]

core::iter seems to be on the way out so this may not matter soon.

[thestinger]

The old iterator module is gone, so this can just be done properly for the new adaptors.