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