Use Minio's optimized SHA-256

internal/repository benchmarks on an Intel i7-3770k:

name               old speed      new speed       delta
PackerManager-8     209MB/s ± 1%    291MB/s ± 1%  +38.94%  (p=0.008 n=5+5)
SaveAndEncrypt-8    112MB/s ± 1%    135MB/s ± 1%  +20.25%  (p=0.008 n=5+5)

cmd/restic/cmd_stats.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"context"
-	"crypto/sha256"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -11,6 +10,8 @@ import (
 	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/restic"
 	"github.com/restic/restic/internal/walker"
+
+	"github.com/minio/sha256-simd"
 	"github.com/spf13/cobra"
 )