Optimize Collections with tinyvec

[appcypher]

@zeeshanlakhani pointed out an opportunity to use tinyvec for a stack structure in a review.

We use working stacks and other collections in a few places that can be optimized with tinyvec. tinyvec keeps the collection on the stack until the static limit is hit subsequently moving the collection to the heap. For some of these structure we know the upper bound that we are likely not to exceed allowing us to keep things mostly on the stack until we hit an exceptional case.

Here are some areas in the library that can be improved:

• Node<K, V, H>
• PathNodes
• previous
• PrivateNodeOnPathHistory
• get_path_nodes