Add UUIDArray type

Also change UUID.Set() to convert nil to NULL in order for
UUIDArray.Set() to support converting [][]byte slices that contain nil.

pgtype/pgtype.go

@@ -49,6 +49,7 @@ const (
 	NumericOID          = 1700
 	RecordOID           = 2249
 	UUIDOID             = 2950
+	UUIDArrayOID        = 2951
 	JSONBOID            = 3802
 )
 
@@ -223,6 +224,7 @@ func init() {
 		"_text":        &TextArray{},
 		"_timestamp":   &TimestampArray{},
 		"_timestamptz": &TimestamptzArray{},
+		"_uuid":        &UUIDArray{},
 		"_varchar":     &VarcharArray{},
 		"aclitem":      &ACLItem{},
 		"bool":         &Bool{},