bookmark: add methods to binary encode and decode bookmark values
Coming new bundle2 parts related to bookmark will use a binary encoding. It
encodes a series of '(bookmark, node)' pairs. Bookmark name has a high enough
size limit to not be affected by
issue5165. (64K length, we are well covered)
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}