WIP: Support attributes as types

[MarcCote]

A couple of highlights:

• Can now define multiple alternatives for a given alias. For instance,

reachable(x) = in(x, I);
reachable(x) = at(x, r) & at(P, r);

• Better expansion of rules, predicates, and aliases for all descendant subtypes.
• While doing the expansion, preserve rules, predicates and alias that are more fine-grained (e.g. keep the definition of open(door) when expanding open(openable) where door is a subtype of openable).
• Add argument types to rule name. This makes finding expanded rules easier.
• Add box, a type of portable container;
• Add stool, a type of portable supporter;
• Now, a fixed in place container is known as chest
• Now, a fixed in place supporter is known as table

TODO

• Tests still failing.
• I'm not entirely sure the "keep fine-grained definitions" work as expected. For instance, I'm not dealing when two parent types defined a same rule/predicate/alias.
• Add documentation and clean commented code.