Change `Dictionary`'s internal data structure to trie

[dahlia]

Currently Dictionary is a thin wrapper around ImmutableDictionry<K, V> (data structure wise). However, it does not share common data between copies, which is highly inefficient both space-wise and time-wise. I believe the memory consumption of Bencodex can be significantly reduced by changing its internal data structure to a trie which shares mutual data between instances rather than a naive hash map. As it would require much less memcpy, I expect it will be even faster than as is.

[dahlia]

Possibly useful projects:

• https://github.com/tunnelvisionlabs/dotnet-trees
• https://github.com/stevenguh/ImmutableTrie

Further reading: dotnet/runtime#26001