cborutil: add a buffering decoder The sansiodecoder leaves it up to the callers to feed in data that wasn't fully consumed last time. This commit implements a decoder that performs buffering of leftover chunks from the previous invocation. It otherwise behaves identically to sansiodecoder. Differential Revision: https://phab.mercurial-scm.org/D4434

cborutil: remove readindefinitebytestringtoiter() This was implemented as part of implementing streaming encoding. It was never used outside of tests. Now that we have a full CBOR decoder, it can be used for incremental decoding of indefinite-length byte strings. This also removes the last use of the vendored cbor2 package from this module. Differential Revision: https://phab.mercurial-scm.org/D4433