Type erasure needed for TableAccess trait and all table "iter" methods

[molpopgen]

Several interfaces return objects implementing Iterator. Currently, the functions return a type for which Iterator is implemented, giving the mistaken impression that there's a struct whose implementation details need to be considered in client code. For most of these, however, all client code needs to worry about is that they impl Iterator. (In fact, I think this is true for all of them...)

We can make these types pub(crate), preventing the names from leaking from the library. Change the function declarations to reflect returning impl Iterator<type=X> (or Box<dyn ..> when the function is declared in a trait).

These changes won't break API as the interface remains unchanged.

[juntyr]

+1 (and if you want to return impl Trait from inside a trait, you can do an … imperfect workaround. You can return an associated type which is then set to impl Trait inside the implementations)

[molpopgen]

Removing this from the next release milestone. It is tedious to get right. There's a bunch of lifetime elision to worry about.

EDIT: we can also kick this down the road because, if done right, it won't break API.