Use binary prefixes for size and rate units

Includes adding support for additional size input suffix Mi and MiB, treated equivalent to M.
Extends binary suffix output with letter i, e.g. Ki and Mi.
Centralizes creation of bit/byte unit strings.

vfs/vfscache/downloaders/downloaders.go

@@ -593,7 +593,7 @@ func (dl *downloader) _stop() {
 
 	// stop the downloader by stopping the async reader buffering
 	// any more input. This causes all the stuff in the async
-	// buffer (which can be many MB) to be written to the disk
+	// buffer (which can be many MiB) to be written to the disk
 	// before exiting.
 	if dl.in != nil {
 		dl.in.StopBuffering()