eolian_mono: fix non-beta build and support for slice/rw_slice keywords

eolian_mono: fix non-beta build and support for slice/rw_slice keywords

Summary:
Add support for `slice` and `rw_slice` keywords in eolian_mono.
Now it generates the proper manual binding type for these native types.
Make unit tests use these keywords instead of native names.

Remove some unnecessary `@beta` tags from eolian types in unit tests so now
tests correctly compile without the `mono-beta` compilation flag.

Also make tests that use `Eina.Binbuf` "beta only".
In a future update, the external type `Eina.Binbuf` will be marked as beta, so
we shield the binding in anticipation.

Test Plan: `meson -Dbindings=mono -Dmono-beta=true` and `meson -Dbindings=mono -Dmono-beta=false`

Reviewers: lauromoura, felipealmeida, q66, segfaultxavi, bu5hm4n

Reviewed By: lauromoura

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9238