revlog: replace revlog._io.size with a new revlog.index.entry_size The `revlogio` class is mostly a relic from the past. Once in charge of the full revlog related Input/Output code, that class gradually lost responsibilities to the point where more IO are now done by `revlog.index` objects or revlog objects themself. I would like to ultimately remove the `revlogio` class, to do so I start simple with move the "entry size" information on the index. (The index is already responsible of the binary unpacking, so it knows the size. Differential Revision: https://phab.mercurial-scm.org/D10309