Implement `FromIterator` for `PathBuf`

[yoshuawuyts]

We currently can't implement the last example from the path submodule. This is because we don't implement FromIterator for our PathBuf struct:

// ... but push is best used if you don't know everything up
// front. If you do, this way is better:
let path: PathBuf = ["c:\\", "windows", "system32.dll"].iter().collect();

We should implement FromIterator for PathBuf so we can collect the way it's done in std. This is fine to have in addition to FromStream because it mostly extends what we're able to collect from already without overlapping.

Marking this as "good first issue" because it should mostly require copying over std's definition. Thanks!