Update does full reload

I've added stone tasks to reload not only prims, but also filetree or src/smalltalk into an existing stone. I have also changed the update.sh script to attempt to reload everything when an existing maglev stone is found.

The reason is that before `update.sh` would just create a new stone and reload_prims if it found an existing extent, which may not be enough if smalltalk code changed as well, but the failure would just confuse users. Since reload_prims clears the PERSISTENT_ROOT, anyway, I guess we can just as well reload all the Smalltalk code, too.

I've tried this locally and it seems to be useful when making changes to src/smalltalk or src/packages (with the obvious problem that for code in src/smalltalk, method removal doesn't work). Maybe @monty or @pbm know if there are undesired side-effects in this change, that we do not want to go in, though.