[R6RS] Hash table terminology consistency

[Michael Sperber]

Another issue wrt hash tables:

- It seems equal-hash is meant to go with equal? as an equivalence
  thingy, but that's never spelled out.  Also, it seems way
  underspecified to me.  (Does it descend into records?  Deal with
  cycles?  Etc.)

- It might be worth spelling out the equivalence thingies for
  `string-hash' and `string-ci-hash' as well.

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla