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)

internal/repository/packer_manager.go

@@ -2,7 +2,6 @@ package repository
 
 import (
 	"context"
-	"crypto/sha256"
 	"os"
 	"sync"
 
@@ -14,6 +13,8 @@ import (
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/fs"
 	"github.com/restic/restic/internal/pack"
+
+	"github.com/minio/sha256-simd"
 )
 
 // Saver implements saving data in a backend.