Simplify should not allocate memory for the idmap

[molpopgen]

Right now, we allocate a new vector each time. This is not efficient and can matter in the forward simulation case.

Instead:

• The structs should store the vector
• resize it if idmap == true
• Return Option<[NodeId]> instead of the vector.

This proposal is a breaking change.

[molpopgen]

Closed in #316