Deletes the log's day folders older than older_than (the log is kept
small by deleting whole days: contract/calls.md, The folder). Before a
day goes, every rated call in it is kept: the call, every call of its tree
(a program's steps), every call its saw names (the earlier turns a row
asks again) and their ratings are copied into one file at the folder's
top level, which every language's reader reads. A row of rated() made
before pruning is the same after.