Fix TR class support for new class expansion

Also add a type for `check-not-unsafe-undefined` which shows
up in the expanded code now.
mzlib/unit200: fix for `undefined`
avoid compiler warnings
fix test for `check-not-unsafe-undefined`
racket/sandbox: fix use-before-definition bug
racket/shared: fix test
Remove the mzlib/class100 library.

Uses of mzlib/class100 have already been removed. The library's
removal was promised in the release notes for Racket v5.3.2.
Update HISTORY file for mzlib/class100 removal
optimizer repair related to use-before-defined

Don't optimize away a use-before-definition in an early compiler
pass. (More specifically, adjust the meaning of a flag within an
optimization helper function so that it works for decisions before the
letrec-fixing pass.)
adjust errortrace to record potential use-before-definition sites

Formerly, reference to a local variable could never raise an
exception.
fix use-before-definition bug in HtDP language implementation

Closes PR 14447
fix typo
typo
Add FAQ section
racket/draw Cocoa: fix surroate-pair patch for Pango
Mark class support as experimental in the TR docs

Please merge to v6.0.1
Demonstrate Racket/Datalog interop more and add unsyntax
Fix shapes with borders to work with pen styles.
Fix TR reference docs for `for`

Please merge to v6.0.1
repair props