Topological order

The idea is to sort addins in a topological order. Each addin can define one of five things for _order_:
* `>X` - I must be immediately after the addin with id X

* `>>X` - I must be somewhere after the addin with id X (but not necessarily immediately) 

* `<X`- I must be immediately before the addin with id X

* `<<X` - I must be somewhere before the addin with id X (but not necessarily immediately) 

* `Number` - A numerical order which is used if none of the above is applicable


optimization is cute, but is it necessary? i optimize where it matters. you add a (very small) amount of code but i think anything that "hides" the actual point of the function and makes it less concise should be discarded. 
i don't know your use case. just my opinion.