refactor(tvix/nar-bridge): combine writers/readers

We can drop most of Hasher if we use a MultiWriter writing to the hash
function and a minimal CountingWriter.

This should make things a bit more understandable.

Change-Id: I37ee72d9a5c73f253aecc1ad761cb723389b89fc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9529
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/nar-bridge/pkg/importer/counting_writer.go

@@ -0,0 +1,21 @@
+package importer
+
+import (
+	"io"
+)
+
+// CountingWriter implements io.Writer.
+var _ io.Writer = &CountingWriter{}
+
+type CountingWriter struct {
+	bytesWritten uint64
+}
+
+func (cw *CountingWriter) Write(p []byte) (n int, err error) {
+	cw.bytesWritten += uint64(len(p))
+	return len(p), nil
+}
+
+func (cw *CountingWriter) BytesWritten() uint64 {
+	return cw.bytesWritten
+}