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.
Closes#8311Closes#6427 (I know it's not really the solution asked for in #6427, sorry!)