Bug fix for branched Core

The range for the splitAt was wrong, creating only a single branch
which then coudldn't be used for rebase.
Testing function to get failure cases

Use failureWorld >>= saveWorld to save a failed rebase case
to disk.
Changed generator for Files to be easily viewable

Gen Char (and therefore Gen String) was generating annoying
characters like \t and other non-standard characters. Changed
to only generator a-Z which may not be ideal.

Should discuss this, but was easier on my eyes in debugging