build-manifest: calculate checksums lazily and in parallel

This commit improves the way build-manifest calculates the checksums
included in the manifest, speeding it up:

* Instead of calculating all the hashes beforehand and then using the
  ones we need, the manifest is first generated with placeholder hashes,
  and then a function walks through the manifest and calculates only the
  needed checksums.

* Calculating the checksums is now done in parallel with rayon, to
  better utilize all the available disk bandwidth.

* Calculating the checksums now uses the sha2 crate instead of the
  sha256sum CLI tool: this avoids the overhead of calling another
  process, but more importantly uses hardware acceleration whenever
  available (the CLI tool doesn't support it at all).

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