Custom compile-time functions (Continuation of #19)

[VonTum]

• Updating flattening such that the instructions within const are all generative
• Updating abstract typecheck to pass over const (not a big deal, since everything's shared in LinkInfo)
• Updating Execute, such that it can call other execute instances recursively to evaluate constants
• Updating tree-walk to show the content of const (and maybe struct too, if we're editing that anyhow, most code would be shared)