Support higher-level TableCollection designs

[molpopgen]

#249 brings up a bigger-picture issue.

The current API closely mimics that of the C library. One could argue that this is too low-level.
A rustier approach would be for client apps to use newtypes representing table collections.
We could provide procedural macros to associate metadata types with these newtypes, eliminating the need for naming types in the metadata retrieval functions.

The same basic tricks could be applied to newtype wrappers for tree sequences, too.