Add pgtype.HstoreArray

This required restructuring array types to lookup oid of element instead of
hard-coding it due to hstore having a variable oid.

pgtype/float4_array.go

@@ -238,10 +238,6 @@ func (src *Float4Array) EncodeText(ci *ConnInfo, w io.Writer) (bool, error) {
 }
 
 func (src *Float4Array) EncodeBinary(ci *ConnInfo, w io.Writer) (bool, error) {
-	return src.encodeBinary(ci, w, Float4Oid)
-}
-
-func (src *Float4Array) encodeBinary(ci *ConnInfo, w io.Writer, elementOid int32) (bool, error) {
 	switch src.Status {
 	case Null:
 		return true, nil
@@ -250,10 +246,15 @@ func (src *Float4Array) encodeBinary(ci *ConnInfo, w io.Writer, elementOid int32
 	}
 
 	arrayHeader := ArrayHeader{
-		ElementOid: elementOid,
 		Dimensions: src.Dimensions,
 	}
 
+	if dt, ok := ci.DataTypeForName("float4"); ok {
+		arrayHeader.ElementOid = int32(dt.Oid)
+	} else {
+		return false, fmt.Errorf("unable to find oid for type name %v", "float4")
+	}
+
 	for i := range src.Elements {
 		if src.Elements[i].Status == Null {
 			arrayHeader.ContainsNull = true