eolian_csharp: add indentation context

eolian_csharp: add indentation context

Summary:
Also, use new context class for cleaner constructs.

Also, make functions receive context objects by reference to avoid
unnecessary object copies (since context objects are bigger now).

This commit contains preparation structures for a future overhaul of
white space generation.

Depends on D8467

Test Plan: ninja test

Reviewers: felipealmeida, lauromoura

Reviewed By: lauromoura

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8468