Move changes from exp2 to main

This is largely a move to simplify a lot of the readers
Also further breaks out functions.

internal/decompress/xz.go

@@ -0,0 +1,14 @@
+package decompress
+
+import (
+	"io"
+
+	"github.com/therootcompany/xz"
+)
+
+type Xz struct{}
+
+func (x Xz) Reader(r io.Reader) (io.ReadCloser, error) {
+	rdr, err := xz.NewReader(r, 0)
+	return io.NopCloser(rdr), err
+}