Add vendor to improve building speed.

This also adds ability to be built in network-constrained environment.

vendor/golang.org/x/crypto/sha3/shake_generic.go

@@ -0,0 +1,19 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !gc || purego || !s390x
+
+package sha3
+
+// newShake128Asm returns an assembly implementation of SHAKE-128 if available,
+// otherwise it returns nil.
+func newShake128Asm() ShakeHash {
+	return nil
+}
+
+// newShake256Asm returns an assembly implementation of SHAKE-256 if available,
+// otherwise it returns nil.
+func newShake256Asm() ShakeHash {
+	return nil
+}