Adding support for Ltree binary encoding

When used with the COPY protocol, the ltree type would currently result
in errors such as:

```
ERROR: unsupported ltree version number 65 (SQLSTATE XX000)
```

This is an attempt to fix this problem by properly adding a byte prefix
to the ltree string as implemented by `ltree_send` and `ltree_receive`
from the PostgreSQL source:
https://github.com/postgres/postgres/blob/master/contrib/ltree/ltree_io.c#L202
https://github.com/postgres/postgres/blob/master/contrib/ltree/ltree_io.c#L226

Author: AmineChikhaoui

ltree.go

@@ -0,0 +1,58 @@
+package pgtype
+
+import (
+	"database/sql/driver"
+)
+
+type Ltree Text
+
+func (dst *Ltree) Set(src interface{}) error {
+	return (*Text)(dst).Set(src)
+}
+
+func (dst Ltree) Get() interface{} {
+	return (Text)(dst).Get()
+}
+
+func (src *Ltree) AssignTo(dst interface{}) error {
+	return (*Text)(src).AssignTo(dst)
+}
+
+func (src Ltree) EncodeText(ci *ConnInfo, buf []byte) ([]byte, error) {
+	return (Text)(src).EncodeText(ci, buf)
+}
+
+func (src Ltree) EncodeBinary(ci *ConnInfo, buf []byte) ([]byte, error) {
+	switch src.Status {
+	case Null:
+		return nil, nil
+	case Undefined:
+		return nil, errUndefined
+	}
+	buf = append(buf, 1)
+	return append(buf, src.String...), nil
+}
+
+func (Ltree) PreferredResultFormat() int16 {
+	return TextFormatCode
+}
+
+func (dst *Ltree) DecodeText(ci *ConnInfo, src []byte) error {
+	return (*Text)(dst).DecodeText(ci, src)
+}
+
+func (dst *Ltree) DecodeBinary(ci *ConnInfo, src []byte) error {
+	return (*Text)(dst).DecodeBinary(ci, src)
+}
+
+func (Ltree) PreferredParamFormat() int16 {
+	return TextFormatCode
+}
+
+func (dst *Ltree) Scan(src interface{}) error {
+	return (*Text)(dst).Scan(src)
+}
+
+func (src Ltree) Value() (driver.Value, error) {
+	return (Text)(src).Value()
+}

pgtype.go

@@ -84,6 +84,7 @@ const (
 	TstzrangeArrayOID   = 3911
 	Int8rangeOID        = 3926
 	Int8multirangeOID   = 4536
+	LtreeOID            = 16407
 )
 
 type Status byte
@@ -327,6 +328,7 @@ func NewConnInfo() *ConnInfo {
 	ci.RegisterDataType(DataType{Value: &Varbit{}, Name: "varbit", OID: VarbitOID})
 	ci.RegisterDataType(DataType{Value: &Varchar{}, Name: "varchar", OID: VarcharOID})
 	ci.RegisterDataType(DataType{Value: &XID{}, Name: "xid", OID: XIDOID})
+	ci.RegisterDataType(DataType{Value: &Ltree{}, Name: "ltree", OID: LtreeOID})
 
 	registerDefaultPgTypeVariants := func(name, arrayName string, value interface{}) {
 		ci.RegisterDefaultPgType(value, name)
@@ -973,6 +975,7 @@ func init() {
 		"jsonb":          &JSONB{},
 		"line":           &Line{},
 		"lseg":           &Lseg{},
+		"ltree":          &Ltree{},
 		"macaddr":        &Macaddr{},
 		"name":           &Name{},
 		"numeric":        &Numeric{},