Make FFinstance instance a global variable

[CarterLi]

We pass instance anywhere, and half of them are FF_UNUSED.

Thoughts? @LinusDierheimer

[LinusDierheimer]

Currently it makes sense, yes. But if we are going to make a library one day, i think having an instance variable is a better design.

[CarterLi]

if we are going to make a library one day, i think having an instance variable is a better design

If we make a library, we should better split FFinstance into smaller structures. It's way too bloated.