Explicitly bind trait method generics

[Nadrieril]

The correspondance between the pair "(impl generics, method generics)" and the full list of generics that must be passed to the function item is not trivial: methods may reuse a default method (which requires changing generics), they may be more specific (e.g. have fewer trait bounds or different late-bound lifetimes) than the trait method, arguments may be out of order because charon modified the parameter list (e.g. for #127).

To support these cases, this PR adds explicit binders to methods in trait decls and trait impls. A method binder binds exactly the generics expected from the method declaration. This works as a map from method generics to a full list of generics to be passed to the bound function item. To map the other way around (from the function item generics to the correponding trait/impl generics and method generics), see in ItemKind.

This is progress towards #180 and #127.