codegen system function signatures so we can use them to register functions

[holic]

it's easy to miss that when you add a function to a system (like in the core module) you also need to make sure that function is registered on the world

we could automate some of this with codegen

[holic]

worked around this for now by centralizing where the function signatures are defined so we could test them from the same source array: #2280

we'll do the codegen step later (see #2332)