Use a hash table keyed by foreign function name to save CIF

Save the libffi "CIF" (foreign structure with information prepared
about function) in a hash table keyed by the foreign function name; if
a function is in the hash table, it is retrieved on call to
#'prepare-function, otherwise it is generated, saved in the table, and
returned.  Information may be removed from the table with
#'unprepare-function.  Start writing #'callable-function which will
generate a lambda that can be called on Lisp arguments and calls
#'call with the CIF and the arguments.