macros: refactor codegen to remove PyMethodDefType

[davidhewitt]

Just a bit of playing around to remove PyMethodDefType by making types in pyo3-macros-backend a bit more concrete and growing the PyClassItems struct to have explicit class_attributes, getters and setters fields. Think overall it leads to simpler generated code which is always nice.

As it is, the macro code changes on this branch are a bit messy - introduces a lot of types which are almost the same and the Vec<(Vec<&'a syn::Attribute>, MethodAndMethodDef)> having tuples feels a bit untidy.

Don't think this is very important to proceed with at all as it was primarily a refactor, so just pushing in case it's interesting for the future. Might close it again before long.