TskWrappedArray life times

[molpopgen]

This type wraps the raw C arrays and is used by NodeIterator objects derived from Tree. However, the type is also public, resulting in a potential safety issue. If the parent Tree is dropped, the array content is invalidated.

Potential solutions:

1. Make the arrays private
2. Try to figure out lifetime annotations.

[molpopgen]

The correct solution is to drop the array wrapper type and use idiomatic methods to return rust slices from raw C arrays.