asdf-support: more filename mangling

New functions:

* ASDF::PROTOBUF-MANGLE-NAME: Takes a pathname, returns a string to be
  used as a filename that represents the original path.
  e.g., #P"/foo/bar/baz.quux" becomes "foo-bar-baz-quux"

* ASDF::PROTOBUF-LISPIZE-PATHNAME: Takes a pathname, returns a new
  pathname with the name mangled as described above, and with a lisp
  type.