Include benchmark scripts

Maktaba includes a few performance-critical functions like `maktaba#rtp#Add` that we try to optimize with caching, etc., but we somehow completely broke the caching at some point and didn't notice. We should add some benchmark scripts to the repo so it's more convenient to check for performance changes.

For example, we want something to start up vim with `--startuptime` and do some heavy `&runtimepath` manipulation using `maktaba#rtp#Add`, `maktaba#rtp#Remove` and friends.