In any case I think the best "key" for a particular definition should
be as simple as `srfi-x--<identifier>` (note the double `-` separating
the SRFI identifier and the definition identifier.

Or any other key that can be automatically derived given the library
and definition identifiers.  (This process also has to be reversible,
i.e. having the key, deriving the library and definition identifier
without any additional lookup.)