Rename Invert to Flip - Issue 10632

Renamed the invert() function in iter.rs to flip().

Also renamed the Invert<T> type to Flip<T>.

Some related code comments changed. Documentation that I could find has
been updated, and all the instances I could locate where the
function/type were called have been updated as well.
Update flip() to be rev().

Consensus leaned in favour of using rev instead of flip.