csharp: Adding ToString methods to Strbuf and custommarshaler.

csharp: Adding ToString methods to Strbuf and custommarshaler.

Summary:
WIN32 should use a allocator and deallocator different from EFL, sometimes, when
freeing a pointer, it should use win32_free. To stardardize, A custommarshaler
is used to fix this problem.

Fixes T8201

Reviewers: lauromoura, felipealmeida

Reviewed By: lauromoura

Subscribers: cedric, brunobelo, felipealmeida, #reviewers, lauromoura, #committers

Tags: #efl

Maniphest Tasks: T8201

Differential Revision: https://phab.enlightenment.org/D9842