This commit adds a data structure, 'RetryLog', which is like a difference list
for the 'Progress' type, except that it only supports efficient appends at
failures. Since the solver continually appends logs and calls 'tryWith' while
exploring the search tree, it is important for those operations to be efficient.
Afterwards, the solver converts the 'RetryLog' back to a 'Progress' so that it
can be processed with pattern matching in Log.hs and Message.hs.