Allow types to specify preference format result and param formats

This will be useful for array and composite types that may have to
support elements that may not support binary encoding.

It also is slightly more convenient for text-ish types to have a default
format of text.

enum_type.go

@@ -128,6 +128,10 @@ func (src *enumType) AssignTo(dst interface{}) error {
 	return errors.Errorf("cannot decode %#v into %T", src, dst)
 }
 
+func (enumType) PreferredResultFormat() int16 {
+	return TextFormatCode
+}
+
 func (dst *enumType) DecodeText(ci *ConnInfo, src []byte) error {
 	if src == nil {
 		dst.status = Null
@@ -152,6 +156,10 @@ func (dst *enumType) DecodeBinary(ci *ConnInfo, src []byte) error {
 	return dst.DecodeText(ci, src)
 }
 
+func (enumType) PreferredParamFormat() int16 {
+	return TextFormatCode
+}
+
 func (src enumType) EncodeText(ci *ConnInfo, buf []byte) ([]byte, error) {
 	switch src.status {
 	case Null: