Separate streaming stuff

The banyan crate itself should be solely about the trees, not about how to map performing operations on trees on threads.

So take out all stream fns and have the mapping from iterators to streams and the threading concept in a separate crate.

Review the banyan crate's stream functions and its mapping from iterators to streams, then identify the threading-related parts described in the issue. Done means the banyan crate contains only tree functionality, while streaming, iterator-to-stream mapping, and threading are provided by a separate crate.

索引モデルが issue の本文から書いたものです。