Update to a more recent version of md5.lisp, taken from SBCL's SB-MD5
contrib. The change is motivated by the fact that SBCL generates a runtime
error "function declared not to return returned" with the current
code. Made a few changes to make the code compile in other Common Lisp
implementations.

darcs-hash:c434dc3b54088edf030c3909c5f03080c396f9d4