Params key in nested model operations is obtained messily

[sudoremo]

Currently, when nesting model operations, a param_key has to be derived in order to assemble the parameters for the sub-operation. Consider the following:

class Operations::User::Create
  model User
  nest_model_op :group, Operations::Group::Create
end

class Operations::Group::Create
  model Group
end

In this case, the operation Operations::Group::Create must receive its parameters as, for example:

#   ┌ param_key
{ group: { name: 'My group' } }

The param is currently derived from the module the sub-operation resides in, in this example:

#                    ┌ param_key (downcased and underscored)
Operations::Group::Create

While this works if the standard naming schemes are used, it is certainly a very suboptimal solution. The correct solution is to use model.model_name.param_key instead. This should generally not break anything, assuming that all operation modules follow the naming conventions.