Add a new deduction to Easy level, which is as small as I can make it
to have the effect of enabling large Easy-level grids to be
constructed in all grid types. Without this, some generations at Easy
level (e.g. 'loopy --generate 1 7x7t9de') can spin forever because
_even with all clues filled in_ the generated grids can't be solved at
that level.

[originally from svn r9143]