revlog: move entry writing in the inner object This is the second big piece of API we want to live on the inner object. With this the inner object have freedom it needs to implement things internally. (except for the delayed write feature of the changelog, that will be migrated in coming changesets)

revlog: move `sidedata` in the inner object Same object as rawtext, but for sidedata.

revlog: move the `rawtext` method on the inner object This is one of the current core goal of the inner object: having a "simple" API for some of the core revlog action, like getting a raw text, with the inner object able to do that part internally, on its own.