Include benchmark scripts

[dbarnett]

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.