eolian: add support for inlist structs

eolian: add support for inlist structs

This adds support for inlist structs, a special type of struct
that can only be used with inlists. This differs from regular
structs in a couple ways:

1) They are stored separately. Just like structs, enums, aliases
   have their own storage, so do inlist structs.
2) They can't be @extern, nor they can be opaque.
3) They are their own type of typedecl.
4) When they contain only one field, this field must be a value
   type always, cannot be a pointer.

Like regular structs, they can have arbitrary fields, and they
can have a pre-set free function via @free().

In C, the inlist structs will be generated exactly like ordinary
ones, except they will have EINA_INLIST before the first field.
Other binding generators can deal with them as they wish, for
example to provide high level interfaces to them.

This does not yet do the plumbing necessary to hook these into
the type system, nor it adds generator support.

@feature