refactor($interpolate): split .parts into .expressions and .separators

BREAKING CHANGE: the function returned by $interpolate
no longer has a `.parts` array set on it.
It has been replaced by two arrays:
* `.expressions`, an array of the expressions in the
  interpolated text. The expressions are parsed with
  $parse, with an extra layer converting them to strings
  when computed
* `.separators`, an array of strings representing the
  separations between interpolations in the text.
  This array is **always** 1 item longer than the
  `.expressions` array for easy merging with it