util/jsonutil: new package

The cornerstone API is a more memory-efficient Unmarshal.
The savings come from re-using a json.Decoder.

BenchmarkUnmarshal-8      	 4016418	       288 ns/op	       8 B/op	       1 allocs/op
BenchmarkStdUnmarshal-8   	 4189261	       283 ns/op	     184 B/op	       2 allocs/op

It also includes a Bytes type to reduce allocations
when unmarshalling a non-hex-encoded JSON string into a []byte.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

util/jsonutil/types.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jsonutil
+
+// Bytes is a byte slice in a json-encoded struct.
+// encoding/json assumes that []byte fields are hex-encoded.
+// Bytes are not hex-encoded; they are treated the same as strings.
+// This can avoid unnecessary allocations due to a round trip through strings.
+type Bytes []byte
+
+func (b *Bytes) UnmarshalText(text []byte) error {
+	// Copy the contexts of text.
+	*b = append(*b, text...)
+	return nil
+}