evolve: store the replacements occurred during `hg evolve` at top level This patch starts storing the replacements made during the `hg evolve` command. The current dictionary is not used anywhere but will be used in future as we want to introduce abort functionality, so we want to delay writing of obsmarkers, storing these exchanges in statefile etc.

evolve: make the _solve* functions return a tuple This patch makes _solve* functions return a tuple which will contain information about whether we were able to solve the instability and if yes, then what was the newnode. For more details about tuple, look the fn docs added. There was some refactoring required in next command because of change in return values. This also leds to omitting the wdir update message in case of dry-run which is good.