cborutil: implement support for streaming encoding, bytestring decoding The vendored cbor2 package is... a bit disappointing. On the encoding side, it insists that you pass it something with a write() to send data to. That means if you want to emit data to a generator, you have to construct an e.g. io.BytesIO(), write() to it, then get the data back out. There can be non-trivial overhead involved. The encoder also doesn't support indefinite types - bytestrings, arrays, and maps that don't have a known length. Again, this is really unfortunate because it requires you to buffer the entire source and destination in memory to encode large things. On the decoding side, it supports reading indefinite length types. But it buffers them completely before returning. More sadness. This commit implements "streaming" encoders for various CBOR types. Encoding emits a generator of hunks. So you can efficiently stream encoded data elsewhere. It also implements support for emitting indefinite length bytestrings, arrays, and maps. On the decoding side, we only implement support for decoding an indefinite length bytestring from a file object. It will emit a generator of raw chunks from the source. I didn't want to reinvent so many wheels. But profiling the wire protocol revealed that the overhead of constructing io.BytesIO() instances to temporarily hold results has a non-trivial overhead. We're talking >15% of execution time for operations like "transfer the fulltexts of all files in a revision." So I can justify this effort. Fortunately, CBOR is a relatively straightforward format. And we have a reference implementation in the repo we can test against. Differential Revision: https://phab.mercurial-scm.org/D3303

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