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.
auto merge of #11686 : mankyKitty/rust/rename-invert-to-flip-issue-10632, r=alexcrichton

Renamed the ```invert()``` function in ```iter.rs``` to ```flip()```, from #10632 

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.

This is my first contribution to Rust! Apologies in advance if I've snarfed the 
PR process, I'm not used to rebase.

I initially had issues with the ```codegen``` section of the tests failing, however
the ```make check``` process is not reporting any failures at this time. I think
that was a local env issue more than me facerolling my changes. :)