improve error message for bad attribute values
  closes PR 14340
Report unspecialized comparisons.

Slower than specialized comparisons, and make futures block.
change class/c to keep a vector of wrapper methods, not a hash table

this change is mostly motivated by improving how generics work on
contracted objects.

Also fix dynamic-send

TR's optimizer seems to get angry at the new send expansion, so
disable optimizations auto-language.rkt and insert-large-letters.rkt
(for now)
add forgotten file
Scribble docs: split preprocessor variants into a separate manual
split "scribble-html-lib" package out of "scribble-lib" package
raco pkg: fix strip of package without "info.rkt"
scribblings/html/resource: allow #f renderer for URL conversion
Adjust prose in how-to-doc section of Scribble docs
Add other future/place primitives to TR base-env

Also fixed the type of `place-dead-evt`.
fix interaction of mixins and contracted classes
fix bugs in the interaction between subclasses and external class/c contracts
Add initial prose for define/contract in the Guide

This commit adds a few sections that explain some of
the subtleties of nested contract boundaries.
disable the new class/c implementation for now

the new implementation doesn't provide eq? guarantees
as often as the old one does, so hold off on this until
we can sort that out. Thanks to drdr for pointing out
the problem:

http://drdr.racket-lang.org/28156/pkgs/drracket-pkgs/drracket-test/tests/drracket/teaching-lang-save-file.rkt
edit define/contract boundary discussion
check context closure language requirements properly

closes PR 14346