using the struct-scheme.hpp and the requirements for
EntryID as a guideline. The goal is to move EntryID
over into the support lib, which means we need to get rid
of all direct proc::asset dependencies. Thus, these generic
ID functions shall form a baseline implementation, while
asset::Struct may provide the previously used implementation
through specialisation -- so the behaviour of EntryID will
not change for the structural assets, but we'll get a more
sane and readable default implementation for all other types.
