[R6RS] Last-element tail call for `for-all' and `exists'

[R. Kent Dybvig]

> As we eliminated this for `for-each' as per Ticket #45, I've also
> eliminated it for `for-all' and `exists'.  Please yell if you
> disagree.

I DISAGREE!!!!  (You said to yell.)

It seems appropriate to require for-all and exists to behave like and and
or in returning the values of the last call and to allow tail-recursive
predicates to be written using them.

Kent