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