internals: document CBOR utilization I spoke with some people at Mozilla about CBOR and they advised me that we should be careful about the subset of CBOR we use in order to mitigate security, performance, and compatibility concerns. This commit establishes a document that attempts to formalize our use of CBOR. Its main limitations are on what types are allowed. It explicitly enumerates which types are supported. Notable missing features include: * Indefinite-length arrays and maps * Text strings (bytes all the way) * Floats * Date/time types * Big integers * Use of indefinite-length byte strings for map keys, values in containers. If we have a need for any of these, we can have a discussion about them when the time comes. Differential Revision: https://phab.mercurial-scm.org/D4412