Some more work on ExtractTo

Added Xz compression support
Started testing using a big squashfs fil (particularly the squashfs from an Arch Linux install img)

internal/compression/xz.go

@@ -0,0 +1,47 @@
+package compression
+
+import (
+	"bytes"
+	"io"
+
+	"github.com/ulikunitz/xz"
+)
+
+//Xz is a decompressor for xz type compression
+type Xz struct{}
+
+//Decompress reads the entirety of the given reader and returns it uncompressed as a byte slice.
+func (z *Xz) Decompress(r io.Reader) ([]byte, error) {
+	rdr, err := xz.NewReader(r)
+	if err != nil {
+		return nil, err
+	}
+	err = rdr.Verify()
+	if err != nil {
+		return nil, err
+	}
+	var data bytes.Buffer
+	_, err = io.Copy(&data, rdr)
+	if err != nil {
+		return nil, err
+	}
+	return data.Bytes(), nil
+}
+
+//Compress compresses the given data (as a byte array) and returns the compressed data.
+func (z *Xz) Compress(data []byte) ([]byte, error) {
+	var buf bytes.Buffer
+	wrt, err := xz.NewWriter(&buf)
+	if err != nil {
+		return nil, err
+	}
+	defer wrt.Close()
+	_, err = wrt.Write(data)
+	if err != nil {
+		return nil, err
+	}
+	if err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}