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.