Abstract type wrapping to a macro?

[molpopgen]

The current TableCollection relies on a heap-allocated tsk_table_collection_t stored in a Box. Thus, we have to do all this "maybe uninitialized" stuff. It would be an improvement to take a cue from rust-GSL and offload the necessary boiler plate to a macro:

1. Pass in the type name, "tskit init" function name, and "tskit free" function name.
2. Heap allocate the type.
3. Init.
4. Implement Drop using the free function.
5. Provide boiler plate for owning and non-owning wrappers, etc..

The nice thing is that this can all be done w/o breaking user-facing API and we can re-use the macro for other types.

[molpopgen]

More on this: current use of Box is far superior to malloc. So let's stick with Box.

Question is how to abstract it out properly? There's definitely room for a trait that defines an API. One issue is that tskit init functions can take variable numbers of arguments, which can make macros annoying.

[molpopgen]

The macro idea has a big downside. It is really annoying to get documentation passed through a macro invocation.