cborutil: implement sans I/O decoder The vendored CBOR package decodes by calling read(n) on an object. There are a number of disadvantages to this: * Uses blocking I/O. If sufficient data is not available, the decoder will hang until it is. * No support for partial reads. If the read(n) returns less data than requested, the decoder raises an error. * Requires the use of a file like object. If the original data is in say a buffer, we need to "cast" it to e.g. a BytesIO to appease the decoder. In addition, the vendored CBOR decoder doesn't provide flexibility that we desire. Specifically: * It buffers indefinite length bytestrings instead of streaming them. * It doesn't allow limiting the set of types that can be decoded. This property is useful when implementing a "hardened" decoder that is less susceptible to abusive input. * It doesn't provide sufficient "hook points" and introspection to institute checks around behavior. These are useful for implementing a "hardened" decoder. This all adds up to a reasonable set of justifications for writing our own decoder. So, this commit implements our own CBOR decoder. At the heart of the decoder is a function that decodes a single "item" from a buffer. This item can be a complete simple value or a special value, such as "start of array." Using this function, we can build a decoder that effectively iterates over the stream of decoded items and builds up higher-level values, such as arrays, maps, sets, and indefinite length bytestrings. And we can do this without performing I/O in the decoder itself. The core of the sans I/O decoder will probably not be used directly. Instead, it is expected that we'll build utility functions for invoking the decoder given specific input types. This will allow extreme flexibility in how data is delivered to the decoder. I'm pretty happy with the state of the decoder modulo the TODO items to track wanted features to help with a "hardened" decoder. The one thing I could be convinced to change is the handling of semantic tags. Since we only support a single semantic tag (sets), I thought it would be easier to handle them inline in decodeitem(). This is simpler now. But if we add support for other semantic tags, it will likely be easier to move semantic tag handling outside of decodeitem(). But, properly supporting semantic tags opens up a whole can of worms, as many semantic tags imply new types. I'm optimistic we won't need these in Mercurial. But who knows. I'm also pretty happy with the test coverage. Writing comprehensive tests for partial decoding did flush out a handful of bugs. One general improvement to testing would be fuzz testing for partial decoding. I may implement that later. I also anticipate switching the wire protocol code to this new decoder will flush out any lingering bugs. Differential Revision: https://phab.mercurial-scm.org/D4414