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/lzma.go

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