A nicer implementation of sync.Once

Doesnt require storing the result of the function in a dedicated global
variable with a dedicated getter function

tools/utils/once_test.go

@@ -0,0 +1,24 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"testing"
+)
+
+var _ = fmt.Print
+
+func TestOnce(t *testing.T) {
+	num := 0
+	var G = (&Once[string]{Run: func() string {
+		num++
+		return fmt.Sprintf("%d", num)
+	}}).Get
+	G()
+	G()
+	G()
+	if num != 1 {
+		t.Fatalf("num unexpectedly: %d", num)
+	}
+}