cmd/dist: implement fetch prototype

With the rename of fetch to fetch-object, we now introduce the `fetch`
command. It will fetch all of the resources required for an image into
the content store. We'll still need to follow this up with metadata
registration but this is a good start.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

progress/humaans.go

@@ -0,0 +1,25 @@
+package progress
+
+import (
+	"fmt"
+	"time"
+
+	units "github.com/docker/go-units"
+)
+
+// Bytes converts a regular int64 to human readable type.
+type Bytes int64
+
+func (b Bytes) String() string {
+	return units.CustomSize("%02.1f %s", float64(b), 1024.0, []string{"B", "KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"})
+}
+
+type BytesPerSecond int64
+
+func NewBytesPerSecond(n int64, duration time.Duration) BytesPerSecond {
+	return BytesPerSecond(float64(n) / duration.Seconds())
+}
+
+func (bps BytesPerSecond) String() string {
+	return fmt.Sprintf("%v/s", Bytes(bps))
+}