Name PG types as words

Though this doesn't follow Go naming conventions exactly it makes names more
consistent with PostgreSQL and it is easier to read. For example, TIDOID becomes
TidOid. In addition this is one less breaking change in the move to V3.

pgtype/pgtype_test.go

@@ -47,7 +47,7 @@ func mustClose(t testing.TB, conn interface {
 	}
 }
 
-func mustParseCIDR(t testing.TB, s string) *net.IPNet {
+func mustParseCidr(t testing.TB, s string) *net.IPNet {
 	_, ipnet, err := net.ParseCIDR(s)
 	if err != nil {
 		t.Fatal(err)