push: perform bookmark export in the push function This part is responsible for adding new bookmarks on the remote. Before that, it was done on its own in `commands.push`. The export is still not integrated with the rest of the push process, but at least it now dwells in the right function.

push: add `pushoperation.bkresult` This attribute will be used to carry the result of the bookmark push (when the `push` function will effectively be doing any bookmarks pushes)

push: pass list of bookmark to `exchange.push` Currently stored but not unused. This parameter will control bookmarks explicitly pushed (added to the server if missing).

exchange: import bookmarks as bookmod Using the original names makes it difficult to use `bookmarks` as a variable name.

push: sanitize handling of bookmark push return value Mixing return and assignment does not make sense, let's unify this.

locarepo: remove the `push` method (API) All the logic of this function is in the exchange.push function for some time. We just stop calling `localrepo.push` in `command.push` to have access to more information. Leaving `localrepo.push` in place will let third-party extensions wrap it but it would never be called by `hg push` making the wrapping useless. Therefore, the method is removed so that third-party code fail noisily and get properly upgraded.