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.

evolvestate: rename the file to state.py and class name to cmdstate The evolvestate.py and the evolvestate class in it were introduced to help `hg evolve` grow continue and abort options. But the class and it's API can be easily used to store state information for other state files like we stored the information for grab state in the previous commit. Let's rename the the file and class.