Prunning unused functions and variables
First of all thanks for making such a great tool! 

This is more of a feasibility question than an actual issue: I was curious about how hard would it be to have `build` exclude unused functions (and potentially also variables) when _building_.

I'm sure there are many like me that like to make a library of common functions and reuse it in different games. However, most of the time not all functions are used, which means it is necessary to do some manual cleaning for each game so that only the necessary functions are included (and tokens are saved). 

Besides being really helpful to save tokens I think this feature would also enable the creation of more reusable pico8 libraries, where the library can be arbitrarily large, and users can just invoke what they need, and unused stuff will just get pruned during building.