[R6RS] Hash table terminology consistency

[Anton van Straaten]

In a final reading of the hash table proposal, I noticed that the hash 
table equivalence predicate was being referred to in five different 
ways, which didn't seem to add any value (other than quirkiness).  I've 
committed a version (rev 897) which makes the following replacements:

equality-checking operation => equivalence predicate
equality operation => equivalence predicate
eq? and eqv? equalities => eq? and eqv? predicates
comparison predicate (3 occurrences) => equivalence predicate

Will, in some cases the changed wording was yours, so you might want to 
check that I haven't changed your intended meaning (sorry for the last 
minuteness...)

I've also changed make-hash-table's equivalence predicate parameter name 
from "comparator" to "equiv?", although that name is used in a more 
specific sense in SRFI 85.  If anyone wants to suggest something better, 
I'll change it.  (One advantage of a shorter name than "comparator" is 
that the prototypes no longer wrap over two lines.)

Anton