vec: remove each_const

An Iterator implementation can be made for &const [T] if it turns out
to be necessary for some use case.
vec: remove eachi

replaced by the `enumerate` method from std::iterator
vec: rm old_iter implementations, except BaseIter

The removed test for issue #2611 is well covered by the `std::iterator`
module itself.

This adds the `count` method to `IteratorUtil` to replace `EqIter`.
vec: replace `position` with `iter().position_`
replace vec::find with the IteratorUtil method
auto merge of #7263 : thestinger/rust/iterator, r=graydon