eolian: introduce typed slice types

eolian: introduce typed slice types

Summary:
This adds two new complex types, slice<T> and rw_slice<T>. This
is necessary to make the type useful to bindings, as Eina_Slice
on its own says nothing about what it's carrying and that prevents
useful code from being generated outside of C.

@feature

Reviewers: bu5hm4n, segfaultxavi, lauromoura, cedric

Reviewed By: cedric

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D7980