Refactor latin.c to make it easier to reuse. Instead of client

Refactor latin.c to make it easier to reuse. Instead of client
programs having to clone the latin_solver() function and insert
their own extra deduction routines, they can now just _call_
latin_solver with enough parameters to let it fit its own deductions
into their difficulty framework and call a set of provided function
pointers to do user deductions. Modified Unequal to work in the new
world, of course.

[originally from svn r8791]