[Wilds] Implement a Filesystem::concatenatePaths(...) method

Summary:
Ref T13098. This is mostly for getting sensible results under Windows, hopefully instead of weird mixed paths with both / and \, at least in more cases.

You can pass in several components like array('/path/to/something/', '/thing.c') and they'll be concatenated with exactly one correct separator.

Test Plan: Used this in the new WorkingCopy stuff, which doesn't run yet. 💁

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13098

Differential Revision: https://secure.phabricator.com/D19691