Optimized combo interpreter

The branch implements 

1) a modular combo interpreter

2) a RAM optimized table, that is each element of the table isn't necessarily a combo::vertex, it can be a primitive type or even a combo_tree (so it can hold lists or generally functions).

I did some benckmarks and I could not measure any slowdown with my new implementation, however the RAM needed to hold a table of a primitive type can be substantially less (up to 8x).

I still need to complete the interpreter to handle list and functions but all unit tests pass (which is supposed to reflect the essential functionalities we care about). I also want to have a look at Jade's work while doing so.