syncs: add ShardedMap.Mutate

To let callers do atomic/CAS-like operations.

Updates tailscale/corp#7355

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

syncs/shardedmap_test.go

@@ -41,4 +41,41 @@ func TestShardedMap(t *testing.T) {
 	if g, w := m.Len(), 0; g != w {
 		t.Errorf("got Len %v; want %v", g, w)
 	}
+
+	// Mutation adding an entry.
+	if v := m.Mutate(1, func(was string, ok bool) (string, bool) {
+		if ok {
+			t.Fatal("was okay")
+		}
+		return "ONE", true
+	}); v != 1 {
+		t.Errorf("Mutate = %v; want 1", v)
+	}
+	if g, w := m.Get(1), "ONE"; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+	// Mutation changing an entry.
+	if v := m.Mutate(1, func(was string, ok bool) (string, bool) {
+		if !ok {
+			t.Fatal("wasn't okay")
+		}
+		return was + "-" + was, true
+	}); v != 0 {
+		t.Errorf("Mutate = %v; want 0", v)
+	}
+	if g, w := m.Get(1), "ONE-ONE"; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+	// Mutation removing an entry.
+	if v := m.Mutate(1, func(was string, ok bool) (string, bool) {
+		if !ok {
+			t.Fatal("wasn't okay")
+		}
+		return "", false
+	}); v != -1 {
+		t.Errorf("Mutate = %v; want -1", v)
+	}
+	if g, w := m.Get(1), ""; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
 }