Remove unused image from new website directory

This image was used for part of a previous commit, but
is unused now. For the record, the image attribution
is:

  "West Village H" by Brennan Degan, licensed under CC BY ND 2.0
  http://www.flickr.com/photos/bdegan/301524841
  http://creativecommons.org/licenses/by-nd/2.0/deed.en
Fix indentation on sierpinski example.
file/md5: faster

Use unsafe operations and forced inlining to speed up
MD5 calculation by about x2.25 on 32-bit platforms, and
use all that plus fixnums to speed up by about x7 on
64-bit platforms.

This implementation is still about a factor of 5 off
a C implementation. So, while this was a fun little
experiment, I'll revert this commit and add a binding
to the OpenSSL MD5 implementation, instead.
Revert "file/md5: faster"

This reverts commit cbc1637e56295e2769bd3a599831cff670584fd6.
add openssl/md5

Like `openssl/sha1`, the `openssl/md5` library falls back to the
plain Racket `file/md5` implementation if OpenSSL is not available.
plt-web: record directory-to-URL mapping in destination
Allow flvectors/fxvectors in Any contract

Closes PR 14374
Use unicode \alpha in front-page example.

I tested this using BrowserStack on browsers going back to IE 6 on Windows XP.
It worked everywhere, even though lots of other parts of the (old) web pages didn't.
Use "explain" on the explanation button for the short front-page examples.
Always use a monospace font, even on old browsers.
Let TR typecheck more un-annotated definitions

This commit allows definitions without type annotations
to refer to definitions later in a module that do have
type annotations. For example,

  (define (f x) (g 0))
  (: g (-> Integer Integer))
  (define (g x) (add1 x))

Previously, such cases required shuffling type annotations
and/or definitions around to satisfy the type-checker.

Note that typechecking may still depend on ordering when
there are not enough type annotations in the code.

Closes PR 11544
Change TR's "untyped identifier" error messages
One more "undefined identifier" change in TR

For the top-level undefined/missing annotation case
Synthesize types for private fields in classes

Allows type annotations to be left out in some cases
Rewrite TR pass for class top-level expressions

Instead of making separate passes over the expressions,
make a single pass to collect necessary information for
type-checking.

This enables simpler processing of type annotations,
super initialization arguments, and other things.
TR: fix error case when superclass is not a class
Improve TR's check for by-name superclass inits

Make sure to check that by-name inits are actually accepted
by the superclass and produce a good error when they aren't.
TR: better error for invalid super-make-object
remove unused module
new web pages: fix generated doc-site.css