Another Brainfuck sample: a Mandelbrot fractal generator

Makes for a good benchmark...
Let 'merge' optimizer discard null adjustements

In cases where the number of increments cancels out the number of
decrements, just drop the commands completely.
Added new 'zeroCells' optimizer

This optimizer can introduce a new dedicated 'SetCell' instruction to
the program, which sets the current cell to a specific value. This is
useful for constructs like [-] which clearly just sets the current cell
to zero. The optimizer relies on the 'merge' phase being done already
so that we can handle cases like [-++--+-] for free.
New 'fuseAssignAndAdjust' optimization

Setting the current cell to a specific value followed by adjusting its
value is the same as setting it to the final value.