util/lru: add a package for a typed LRU cache

Updates tailscale/corp#7355

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/lru/lru_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lru
+
+import "testing"
+
+func TestLRU(t *testing.T) {
+	var c Cache[int, string]
+	c.Set(1, "one")
+	c.Set(2, "two")
+	if g, w := c.Get(1), "one"; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+	if g, w := c.Get(2), "two"; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+	c.DeleteOldest()
+	if g, w := c.Get(1), ""; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+	if g, w := c.Len(), 1; g != w {
+		t.Errorf("Len = %d; want %d", g, w)
+	}
+	c.MaxEntries = 2
+	c.Set(1, "one")
+	c.Set(2, "two")
+	c.Set(3, "three")
+	if c.Contains(1) {
+		t.Errorf("contains 1; should not")
+	}
+	if !c.Contains(2) {
+		t.Errorf("doesn't contain 2; should")
+	}
+	if !c.Contains(3) {
+		t.Errorf("doesn't contain 3; should")
+	}
+	c.Delete(3)
+	if c.Contains(3) {
+		t.Errorf("contains 3; should not")
+	}
+}