Refine table column design

[molpopgen]

No description provided.

[molpopgen]

TODO:

• Move the trait into src/traits.rs
• Add the trait to the prelude
• Add fns for the rest of the table types.

[molpopgen]

We need to ignore the TODO list above -- it turns out that many/most tables don't even have the slice accessors in place. Those that do are doing it with unsafe code that we can easily push back into the sys module. But all of that should be in a later PR.