fs: use an iterative algorithm for 'mkdir_recursive'

as requested in #6109
fs: use an iterative algorithm for 'rmdir_recursive'

For now, the windows version uses stat, just as before.
We should switch back to lstat as soon as #12795 is closed.
fs: units tests for mkdir_recusive and rmdir_recursive

The rmdir test is blocked by #12795 on windows.
auto merge of #12573 : lbonn/rust/unrecurs, r=alexcrichton

As mentioned in #6109, ```mkdir_recursive``` doesn't really need to use recursive calls, so here is an iterative version.
The other points of the proposed overhaul (renaming and existing permissions) still need to be resolved.

I also bundled an iterative ```rmdir_recursive```, for the same reason.

Please do not hesitate to provide feedback on style as this is my first code change in rust.
std: Rename Chan/Port types and constructor

* Chan<T> => Sender<T>
* Port<T> => Receiver<T>
* Chan::new() => channel()
* constructor returns (Sender, Receiver) instead of (Receiver, Sender)
* local variables named `port` renamed to `rx`
* local variables named `chan` renamed to `tx`

Closes #11765
auto merge of #12815 : alexcrichton/rust/chan-rename, r=brson

* Chan<T> => Sender<T>
* Port<T> => Receiver<T>
* Chan::new() => channel()
* constructor returns (Sender, Receiver) instead of (Receiver, Sender)
* local variables named `port` renamed to `rx`
* local variables named `chan` renamed to `tx`

Closes #11765
rename std::vec -> std::slice

Closes #12702