Add WIM parser

This new code supports parsing WIM files. Currently, only WIM files that
were created with no compression or LZX compression are supported. Xpress
compression is not.

reparse.go

@@ -43,8 +43,12 @@ func (e *UnsupportedReparsePointError) Error() string {
 // DecodeReparsePoint decodes a Win32 REPARSE_DATA_BUFFER structure containing either a symlink
 // or a mount point.
 func DecodeReparsePoint(b []byte) (*ReparsePoint, error) {
-	isMountPoint := false
 	tag := binary.LittleEndian.Uint32(b[0:4])
+	return DecodeReparsePointData(tag, b[8:])
+}
+
+func DecodeReparsePointData(tag uint32, b []byte) (*ReparsePoint, error) {
+	isMountPoint := false
 	switch tag {
 	case reparseTagMountPoint:
 		isMountPoint = true
@@ -52,11 +56,11 @@ func DecodeReparsePoint(b []byte) (*ReparsePoint, error) {
 	default:
 		return nil, &UnsupportedReparsePointError{tag}
 	}
-	nameOffset := 16 + binary.LittleEndian.Uint16(b[12:14])
+	nameOffset := 8 + binary.LittleEndian.Uint16(b[4:6])
 	if !isMountPoint {
 		nameOffset += 4
 	}
-	nameLength := binary.LittleEndian.Uint16(b[14:16])
+	nameLength := binary.LittleEndian.Uint16(b[6:8])
 	name := make([]uint16, nameLength/2)
 	err := binary.Read(bytes.NewReader(b[nameOffset:nameOffset+nameLength]), binary.LittleEndian, &name)
 	if err != nil {