443 - simple graphics primitives

http://docs.racket-lang.org/graphics/Mouse_Operations.html

Like with the text mode primitives, we still don't have a story for
writing white-box tests for code using these.
444 - bring back dispatch based on operand/result types

Was dropped in commit 149. But we need it for more convenient
overloading, especially now that the right way to build tagged-values is
unclear.

The original concern was that type/otype would make code harder to
'assemble' down to native. But we should be able to insert CALL
instructions to the right clause inside a function's code. So keep it
around in the toolbox.