Remove need to malloc a tree sequence

[molpopgen]

We can simplify the initialization:

1. MaybeUninit
2. Call the init function
3. MaybUninit::assume_init()

This removes an indirection and clarifies any safety
documentation: we don't have a pointer and the pub
API to create a treeseq is guaranteed to have called
the initialization code.a

[molpopgen]

Closed by #306