The md5 wrapper.

This is a drop-in replacement for the md5 microcode module and the
md5-* procedures in runtime/crypto.scm.  It is not part of the core
build and can be built outside the core build tree in the customary
way:

    ./configure [--with-openssl=directory]...
    make all check install

The install target copies a shared library shim and compiled Scheme
files into the system library path, and re-writes the optiondb.scm
found there.  You can override the default command name "mit-scheme"
(and thus the system library path) by setting MIT_SCHEME_EXE.

To use: (load-option 'MD5) and import the bindings you want.  They are
not exported to the global environment because they would conflict
with the exports from (runtime crypto).
