Add Composite type for inplace row() values handling

Composite() function returns a private type, which should
be registered with ConnInfo.RegisterDataType for the composite
type's OID.

All subsequent interaction with Composite types is to be done
via Row(...) function. Function return value can be either
passed as a query argument to build SQL composite value out of
individual fields or passed to Scan to read SQL composite value
back.

When passed to Scan, Row() should have first argument of type
*bool to flag NULL values returned from query.

pgtype.go

@@ -167,6 +167,15 @@ func (f BinaryDecoderFunc) DecodeBinary(ci *ConnInfo, src []byte) error {
 	return f(ci, src)
 }
 
+//The BinaryEncoderFunc type is an adapter to allow the use of ordinary functions as BinaryDecoder types.
+// If f is a function with the appropriate signature, BinaryEncoderFunc(f) is a BinaryDecoder that calls f.
+type BinaryEncoderFunc func(ci *ConnInfo, buf []byte) ([]byte, error)
+
+// EncodeBinary calls f(ci, buf)
+func (f BinaryEncoderFunc) EncodeBinary(ci *ConnInfo, buf []byte) (newBuf []byte, err error) {
+	return f(ci, buf)
+}
+
 var errUndefined = errors.New("cannot encode status undefined")
 var errBadStatus = errors.New("invalid status")