Searches a list of pairs for a pair whose second component equals a specified
rev_assoc y [(x1,y1),...,(xn,yn)] returns the first (xi,yi) in the list
such that yi equals y. The lookup is done on an eqtype, i.e., the SML
implementation must be able to decide equality for the type of y.
Fails if no matching pair is found. This will always be the case if the list