programs.systemtap: add nixos option for installing systemtap

also enables debug feature on kernel

nixos/modules/module-list.nix

···
   ./programs/ssh.nix
   ./programs/ssmtp.nix
   ./programs/sysdig.nix
+  ./programs/systemtap.nix
   ./programs/sway.nix
   ./programs/thefuck.nix
   ./programs/tmux.nix

nixos/modules/programs/systemtap.nix

···
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let cfg = config.programs.systemtap;
+in {
+
+  options = {
+    programs.systemtap = {
+      enable = mkOption {
+        default = false;
+        description = ''
+          Install <command>systemtap</command> along with necessary kernel options.
+        '';
+      };
+    };
+  };
+  config = mkIf cfg.enable {
+    system.requiredKernelConfig = with config.lib.kernelConfig; [
+      (isYes "DEBUG")
+    ];
+    boot.kernel.features.debug = true;
+    environment.systemPackages = [
+      config.boot.kernelPackages.systemtap
+    ];
+  };
+
+}