fix(trie): sparse trie walk should be done in a sorted manner

[shekhirin]

Two fixes:

1. Iterate children of a branch node in reverse, because we push them to the stack and then pop from it. So pushing a..=f means popping f..=a. We need to push f..=a and pop a..=f to make it a DFS search.
2. Return paths in the same order as they were popped from the trie, and it's a sorted order. It's crucial for the prefix set to work efficiently.

[gakonst]

vec with capacity or smallvec?

[shekhirin]

we cannot estimate how many targets will be collected. It depends on the trie structure, so we can't just do 16^N to get the number of nodes at level N.