Adjust class/c to use the new strategy for the external
parts of a contract (field, method, init, absent clauses)

Still to do:

- delay the creation of the class that checks internal contracts
  until a new derived class is created from a contracted class.

- extend the new implementation so it handles interface contracts
  (currently the old version of the external contracts is still there,
  just to handle this case)

The first of those is what (I hope) will bring down the memory
footprint for class/c contracts.
unbreak racket: disable the new class/c in favor of the old one (for now)
scribble/text: finish documentation
scribble/html: first cut at documentation
scribble-{doc,lib}: fix package dependencies
Fix get-custom-snapshots to conform to the docs.
Check for incomplete blame information and error.
Expose missing blame parties to the contract profiler.

Done by pairing up incomplete blame objects with the missing blame party
before attaching it to the continuation mark. Other approaches were
explored (having a separate mark for each, imperatively updating blame
objects to add missing blame parties, etc.), but pairing had the least
overhead.
Fix `for` to propagate source locations in more cases.

Thanks to Tobias Hammer for the patch.

Closes PR 14343.
Fix TR optimizer tests for new source locations.
Fix doc contract for list-box%
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