Add pgtype.Record and prerequisite restructuring

Because reading a record type requires the decoder to be able to look up oid
to type mapping and types such as hstore have types that are not fixed between
different PostgreSQL servers it was necessary to restructure the pgtype system
so all encoders and decodes take a *ConnInfo that includes oid/name/type
information.

pgtype/array.go

@@ -27,7 +27,7 @@ type ArrayDimension struct {
 	LowerBound int32
 }
 
-func (dst *ArrayHeader) DecodeBinary(src []byte) (int, error) {
+func (dst *ArrayHeader) DecodeBinary(ci *ConnInfo, src []byte) (int, error) {
 	if len(src) < 12 {
 		return 0, fmt.Errorf("array header too short: %d", len(src))
 	}
@@ -60,7 +60,7 @@ func (dst *ArrayHeader) DecodeBinary(src []byte) (int, error) {
 	return rp, nil
 }
 
-func (src *ArrayHeader) EncodeBinary(w io.Writer) error {
+func (src *ArrayHeader) EncodeBinary(ci *ConnInfo, w io.Writer) error {
 	_, err := pgio.WriteInt32(w, int32(len(src.Dimensions)))
 	if err != nil {
 		return err