refactor population table storage

[molpopgen]

• refactor pop table to store pointer
• change storage to const ptr, various fixes
• Sneak in design of owning table.
• Passes if we do this:

[molpopgen]

As far as #262 goes:

1. We have to remove lifetime annotation from tables
   and replace held reference with pointer.
2. We have to break API of TableAccess, returning
   &FooTable instead of FooTable<'a>.
   This change will not break any code using
   current implementers.
   Rather, it only breaks API for those
   implementing it, which is realistically
   no-one right now.
3. We almost certainly need FooTable to
   know if they own their pointers or not.

The best plan would be to do 1 and 2 in a PR.
Then stack a PR on top to do 3.
Finally, we can take a shot at #262.

[molpopgen]

Update to the previous:

It probably makes sense to abstract out item 3
before tackling 1 and 2.

[molpopgen]

We have found a way out of needing an "owning" abstraction:

The new table types contain both MBox and the refactored table type.

[molpopgen]

closing -- see comments in #262