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It has been replaced by two arrays: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