Use ast instead of reflection.

[OneOfOne]

[muzea]

Is there any update for this issue? Using reflection is a bit slow for me.
If nobody handles it, I can try it.

BTW, does this project need to add test cases?

[arran4]

I think this would conflict with my latest PR. Using the AST removes the ability to manipulate functions and has some more cases where it could fall over such as when you include structures from other packages.