revlog: move the details of revlog "v0" index inside revlog.utils.constants
the revlog module is quite large and this kind of format information would handy
for other module. So let us start to gather this information about the format in
a more appropriate place.
Differential Revision: https://phab.mercurial-scm.org/D10303
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true