eolian_csharp: hide struct native representation inside the managed struct

eolian_csharp: hide struct native representation inside the managed struct

Summary:
Rework the struct binding generator to declare the native struct nested inside
the managed one.
This way native structs are less likely to cause confusion; for example
with an IDE that supports automatic completion.

Get rid of struct conversion class methods in favor of using (the already
generated) implicit conversion operators.

Depends on D8469

Reviewers: segfaultxavi, lauromoura, felipealmeida

Reviewed By: lauromoura

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8470