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/gzip.go

@@ -0,0 +1,13 @@
+package decompress
+
+import (
+	"io"
+
+	"github.com/klauspost/compress/zlib"
+)
+
+type GZip struct{}
+
+func (g GZip) Reader(src io.Reader) (io.ReadCloser, error) {
+	return zlib.NewReader(src)
+}