Sounds like all you need there is a compiler (people already use minifiers, right?) that would include or inline the used functions as appropriate and the cost

Sounds like all you need there is a compiler (people already use minifiers, right?) that would include or inline the used functions as appropriate and the cost would mainly go away? Or add in the "fetch this version from CDN" library loader code if it's a stdlib we expect to be cached and used by many js scripts.