Rename a few typeck method-related structures to UpperCamelCase.
Replace callee_id with information stored in method_map.
auto merge of #12571 : eddyb/rust/kill-callee-id, r=nikomatsakis

Every method call and overloaded operator had a `callee_id` that was be used to store the method type and type substitutions, that information is now stored in the `method_map`, alongside the method's origin.