eolian_cxx: rework context code, also more flexible scope indentation generator

eolian_cxx: rework context code, also more flexible scope indentation generator

Summary:
Context management classes now store constant copies of the tags they
are supposed to hold. So, they are immutable objects now.
Functions that manipulate context create new object copies as needed.
`constexpr` was added for class constructor and functions.

Indentation generator now used four space by default.
Also, the amount of desired spaces can now be specified at call site.

Added helper methods to return 'indented' and 'unindented' version of a
given indentation generator object.

Test Plan: ninja test

Reviewers: felipealmeida, lauromoura

Reviewed By: lauromoura

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8467