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/database_sql.go

@@ -0,0 +1,66 @@
+package pgtype
+
+import (
+	"bytes"
+	"errors"
+)
+
+func DatabaseSQLValue(ci *ConnInfo, src Value) (interface{}, error) {
+	switch src := src.(type) {
+	case *Bool:
+		return src.Bool, nil
+	case *Bytea:
+		return src.Bytes, nil
+	case *Date:
+		if src.InfinityModifier == None {
+			return src.Time, nil
+		}
+	case *Float4:
+		return float64(src.Float), nil
+	case *Float8:
+		return src.Float, nil
+	case *GenericBinary:
+		return src.Bytes, nil
+	case *GenericText:
+		return src.String, nil
+	case *Int2:
+		return int64(src.Int), nil
+	case *Int4:
+		return int64(src.Int), nil
+	case *Int8:
+		return int64(src.Int), nil
+	case *Text:
+		return src.String, nil
+	case *Timestamp:
+		if src.InfinityModifier == None {
+			return src.Time, nil
+		}
+	case *Timestamptz:
+		if src.InfinityModifier == None {
+			return src.Time, nil
+		}
+	case *Unknown:
+		return src.String, nil
+	case *Varchar:
+		return src.String, nil
+	}
+
+	buf := &bytes.Buffer{}
+	if textEncoder, ok := src.(TextEncoder); ok {
+		_, err := textEncoder.EncodeText(ci, buf)
+		if err != nil {
+			return nil, err
+		}
+		return buf.String(), nil
+	}
+
+	if binaryEncoder, ok := src.(BinaryEncoder); ok {
+		_, err := binaryEncoder.EncodeBinary(ci, buf)
+		if err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
+	}
+
+	return nil, errors.New("cannot convert to database/sql compatible value")
+}