Make comm_p tree building streaming

[vmx]

Description

Currently the comm_p calculation needs an mmap'ed file of the leaves. A tree is then built bottom up.

Instead you could use some input stream and build the tree on the fly as you are really only interested in the root of the tree.

Calculating comm_p is currently used in preparing storage deals and if the IPFS integration for Lotus is used.

Acceptance criteria

comm_p doesn't need the input data as a single file, but can consume it in streaming fashion.

Risks + pitfalls

The questions is what a "stream" is, especially if you want to interact with it over an FFI.

Where to begin

Start with the current way of having a file with all the leaves and stream that into a tree builder. Once that works you can think about how to make the input a stream.