file: Introduce filehasher subcomponent

[nolash]

This PR is part of a series of PRs that introduces an interface that allows chaining of components that receive a data stream and generate hashes and intermediate Merkle-Tree chunks. The individual PR steps will be partitioned from #2022 (branch https://github.com/nolash/swarm/tree/filehasher-avenged ) as follows:

1. Introduce SectionWriter, implement this interface in bmt, make AsyncHasher standalone
2. Move AsyncHasher to file/hasher
3. Add reference implementation of the Filehasher algorithm
4. Add implementation of SectionWriter sub-component for hashing intermediate Merkle Tree levels (this PR)
5. Add implementation of SectionWriter component executing the FileHasher algorithm
6. Add a "splitter" that bridges io.Reader and SectionWriter, and an implementation of SectionWriter component that provides Chunk output.
7. Add implementation of SectionWriter that provides encryption, along with a test utility SectionWriter implentation of a data cache.
8. Evaluate and prune bmt.Hasher exports wtr AsyncHasher

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