salvage some of core::old_iter for core::iter, remove the rest

[thestinger]

All of the adaptors implemented on top of BaseIter should be removed and reimplemented as generic adaptors for any internal iterator. The traits are generally useful, but BaseIter is the wrong place for a size hint to be.

The only adaptors that can really take advantage of the hint are ones that convert to a container with the concept of reserving space. It should be possible to take advantage of the reserve on any iterator with a known size, not just each.

[thestinger]

Nominating for the backwards compatible milestone. I think most of this will end up scrapped and replaced with external iterators and an Iterable trait.

We do need generic internal iterator adaptors if we want good support for them, but there aren't actually any in this module working for anything beyond BaseIter.

[thestinger]

Nothing to salvage, just going to remove it.

[thestinger]

#7334