Micro-optimization for hashing.Writer/PackerManager

name             old time/op    new time/op    delta
PackerManager-8     247ms ± 1%     246ms ± 1%  -0.43%  (p=0.001 n=18+18)

name             old speed      new speed      delta
PackerManager-8   213MB/s ± 1%   214MB/s ± 1%  +0.43%  (p=0.001 n=18+18)

name             old alloc/op   new alloc/op   delta
PackerManager-8    92.2kB ± 0%    91.5kB ± 0%  -0.82%  (p=0.000 n=19+20)

name             old allocs/op  new allocs/op  delta
PackerManager-8     1.43k ± 0%     1.41k ± 0%  -1.67%  (p=0.000 n=20+20)

internal/hashing/writer.go

@@ -15,13 +15,14 @@ type Writer struct {
 func NewWriter(w io.Writer, h hash.Hash) *Writer {
 	return &Writer{
 		h: h,
-		w: io.MultiWriter(w, h),
+		w: w,
 	}
 }
 
 // Write wraps the write method of the underlying writer and also hashes all data.
 func (h *Writer) Write(p []byte) (int, error) {
 	n, err := h.w.Write(p)
+	h.h.Write(p[:n])
 	return n, err
 }