syncs: add ShardedMap type

Updates tailscale/corp#7354

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

syncs/shardedmap_test.go

@@ -0,0 +1,44 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package syncs
+
+import "testing"
+
+func TestShardedMap(t *testing.T) {
+	m := NewShardedMap[int, string](16, func(i int) int { return i % 16 })
+
+	if m.Contains(1) {
+		t.Errorf("got contains; want !contains")
+	}
+	if !m.Set(1, "one") {
+		t.Errorf("got !set; want set")
+	}
+	if m.Set(1, "one") {
+		t.Errorf("got set; want !set")
+	}
+	if !m.Contains(1) {
+		t.Errorf("got !contains; want contains")
+	}
+	if g, w := m.Get(1), "one"; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+	if _, ok := m.GetOk(1); !ok {
+		t.Errorf("got ok; want !ok")
+	}
+	if _, ok := m.GetOk(2); ok {
+		t.Errorf("got ok; want !ok")
+	}
+	if g, w := m.Len(), 1; g != w {
+		t.Errorf("got Len %v; want %v", g, w)
+	}
+	if m.Delete(2) {
+		t.Errorf("got deleted; want !deleted")
+	}
+	if !m.Delete(1) {
+		t.Errorf("got !deleted; want deleted")
+	}
+	if g, w := m.Len(), 0; g != w {
+		t.Errorf("got Len %v; want %v", g, w)
+	}
+}