Support printf formats in terminfo strings

terminfo parameterized strings supports a limited subset of printf-style
formatting operations, such as %#5.3d.
auto merge of #7161 : kballard/rust/terminfo-parm-format, r=thestinger

Introduce support for terminfo's subset of printf-style formatting on doxXs operations.

r? @thestinger
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