Remove deprecated owned vector from iterator example.
auto merge of #14547 : reem/rust/remove-owned-vec-docs, r=alexcrichton

The last example in the containers and iterators guide had a superfluous owned vector in it. Everything works fine without it, so I removed it to avoid confusion.
Add os::split_paths

Adds a platform-specific function, `split_paths` to the `os` module. This
function can be used to parse PATH-like environment variables according to
local platform conventions.

Closes #14352.