Implement a QAST backend

[masak]

<pmurias> masak: re having 007 target js, maybe it would be interesting if I made 007 also target QAST (so it would 
          support js, moarvm and jvm)
<masak> pmurias: most certainly. let me know what I can do to help.
<masak> pmurias: offhand, I'm thinking it might be easier for you if I write the JS backend first -- extending it a 
        second time will be way simpler.
<pmurias> write any backend would make subsequent ones easier as that would help define the line between backend and 
          interpreter
<masak> right

[masak]

Marking as currently-blocked because #166 needs to happen first.