types/structs: add structs.Incomparable annotation, use it where applicable

Shotizam before and output queries:

sqlite> select sum(size) from bin where func like 'type..%';
129067
=>
120216

types/structs/structs.go

@@ -0,0 +1,16 @@
+// 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 structs contains the Incomparable type.
+package structs
+
+// Incomparable is a zero-width incomparable type. If added as the
+// first field in a struct, it marks that struct as not comparable
+// (can't do == or be a map key) and usually doesn't add any width to
+// the struct (unless the struct has only small fields).
+//
+// Be making a struct incomparable, you can prevent misuse (prevent
+// people from using ==), but also you can shrink generated binaries,
+// as the compiler can omit equality funcs from the binary.
+type Incomparable [0]func()