feat($q): add streamlined ES6-style interface for using $q

This potentially helps lead the way towards a more performant fly-weight
implementation, as discussed earlier in the year. Using a constructor means we
can put things in the prototype chain, and essentially treat $q as a Promise
class, and reuse methods as appropriate.

Short of that, I feel this style is slightly more convenient and streamlined,
compared with the older API.

Related to #6427

(This doesn't do what was requested in #6427, however I feel it's a better
solution as it looks similar to the newer native Promise implementations becoming common
in the web platform).
theoretically, if `index` overflows, this block might actually be wrong due to not clearing the fulfillmentHandler/rejectionHandler, but if it is, it's wrong in Bluebird too.