Racket 6.2.1

Version 6.2.1 patches the recent v6.2 release in three small ways:

* For the How to Design Programs teaching languages, DrRacket offers
  an option to use the old style for printing the constants `true`,
  `false`, and `empty` instead of `#true`, `#false`, and `'()`.

* The teaching languages come with some additional functions to match
  the August 2015 stable release of HtDP 2nd edition.

* A repair to the compiler avoids an infinite loop at compile time for
  certain expressions that should loop forever at run time.

Merge git.racket-lang.org:plt