replace some fmt.Sprintfs with strconv

Teeny-tiny optimizations:

    BenchmarkSprintf-10       37735996    32.31  ns/op  0 B/op  0 allocs/op
    BenchmarkItoa-10         591945836     2.031 ns/op  0 B/op  0 allocs/op
    BenchmarkFormatUint-10   593701444     2.014 ns/op  0 B/op  0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cmd/ctr/commands/snapshots/snapshots.go

@@ -23,6 +23,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"text/tabwriter"
 	"time"
@@ -207,7 +208,7 @@ var usageCommand = cli.Command{
 		var displaySize func(int64) string
 		if context.Bool("b") {
 			displaySize = func(s int64) string {
-				return fmt.Sprintf("%d", s)
+				return strconv.FormatInt(s, 10)
 			}
 		} else {
 			displaySize = func(s int64) string {