[jwric]

Every transfer to or from a presigned URL goes through the SDK's tracel-artifact, concurrently and with byte progress. Stacked on #41.

• Uploads. models push and the code versions package uploads send up to eight parts or files at once, streamed from disk instead of read into memory. A code version's binaries for several targets upload together.
• Downloads. models pull and artifacts download fetch a large file in several ranges at once, into a temporary directory beside the destination. Each file's size and checksum are checked before any of them moves in, so a failed download leaves the destination as it was. Paths and existing files are still checked first, and --force still overwrites.
• Progress. On a terminal, one bar shows the bytes moved, the rate and the time left.
• Not-found errors. artifacts download says whether the experiment, the artifact, or its stored files are missing. It used to say "No experiment N" for all three. Any download whose stored files are gone from the Console's storage now says so of what it asked for.
• The CLI's own part planner, part uploader and hashing writer are gone, along with their tests; tracel-artifact does that work and tests it.

Builds against tracel-ai/tracel#203, the concurrent uploads, and tracel-ai/tracel-client#57, which drops the client's URL transfers and adds the not-found codes. Both are pinned by git revision for review.