efl-csharp: Add implicit conversions for Eina.Value
For basic types, this will make it easier to pass Eina.Values into
functions, without requiring to setup and later Set() or Get() calls.
As discussed on irc, this seems to be a better way to improve the Value
C# API than using method chaining.
Test Plan: run tests
Reviewers: segfaultxavi, felipealmeida
Reviewed By: segfaultxavi
Maniphest Tasks: T7388
Differential Revision: https://phab.enlightenment.org/D7526