wireprotov2: define type to represent pre-encoded object
An upcoming commit will introduce a caching layer to command
serving. This will require the ability to cache pre-encoded data.
This commit introduces a type to represent pre-encoded data and
teaches the output layer to not CBOR encode an instance of that
type.
Differential Revision: https://phab.mercurial-scm.org/D4772
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.