`Vector` nodes unnecessarily have heap allocated `Vec`s

[orium]

This is bad because it involves more heap allocations and also more efford when Vecs need to expand.

To not lose the ability to configure the branching factor this is better done when rust-lang/rust#44580 is ready and stable. If we need this before const-generics stabilize then we can use the smallvec crate + some type aliases.

[michaelsproul]

I'm going to have a crack at this, using const-generics for Vector.