Adjust insert-large-letters
Simplify in insert-large-letters
Eliminate require/typed in world helper file
Eliminate redundant cast and assertion in last.rkt
Fix bug in racket/gui
A first attempt at protecting `this`

This closes a hole with (send this m) cases in mixins.
Use `this` protection in more cases

Also reduce the memory overhead by only generating the
this contract at a single point and binding it.
Fix some GUI types