[racket] Dr. Racket for iOS

Some reasons/scenarios I see for doing DrRacket in HTML5 on tablets are:

* Stress-test the Racket HTML5 system: if DrRacket can run, most other 
stuff should.

* Lots of people have tablets, and then they could use DrRacket on them.

* When Racketeers are waiting for the bus or whatever, instead of 
playing another game of Angry Flappy Sudoku Ninja, they could play with 
some idea in DrRacket.  Maybe it is different than what they normally 
work on when at a keyboard, and that can be good.

Most of my early Racket libraries, as well as Quack, were written from 
parks and cafes and such, when I had a modest laptop and no Internet, 
and I was very bored.  Since that laptop had maybe 1/20 the CPU of a 
typical tablet this year, and 1/500 the RAM, maybe some smart person 
could get DrRacket running OK in HTML5 on ordinary tablets, so even more 
people's boredom can be turned into Ideas That Evaluate.

Neil V.