Currently, it's not possible to override MethodCallback in Methods because Methods::merge fails early if a callback with the name already exists

This makes it impossible to disable various callbacks dynamically when RpcModules (via server proc macro's into_rpc) are merged.

now that verify_method_name is public this could be solved by:

• support Methods::iter and Methods::into_iter so callbacks can be inserted individually.
• support Methods::merge_replace which replaces existing handlers