Extend the MsgPack port to store a pointer to a MsgPack context (the
lifetime of a MsgPack port is static, so we simply allocate it on the
stack), avoiding copying. This context can later be used for dumping
MsgPack to Lua, and possibly acquired by a MsgPack object when the
'MsgPack object' mode of `port_dump_lua` is used.

Needed for #8147

NO_CHANGELOG=refactoring
NO_DOC=refactoring
NO_TEST=refactoring