Provide array_interleave()

Summary:
This tackles the specific case of:

phutil_render_tag(
  'div',
  array(),
  implode(' · ', $list_of_stuff));

...replacing it with:

phutil_tag(
  'div',
  array(),
  array_interleave(" \xC2\xB7 ", $list_of_stuff));

While we could continue using implode(), that means we can never get rid of the __toString() call.

Test Plan: unit tests

Reviewers: vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4682