Fix Trac #3245: memoising typeOf

Fix Trac #3245: memoising typeOf

The performance bug in #3245 was caused by computing the typeRep
once for each call of typeOf, rather than once for each dictionary
contruction.  (Computing TypeReps is reasonably expensive, because
of the hash-consing machinery.)

This is readily fixed by putting the TypeRep construction outside
the lambda.  (Arguably GHC might have worked that out itself,
but it involves floating something between a type lambda and a
value lambda, which GHC doesn't currently do. If it happens a lot
we could fix that.)