Cell signedness examples

[cpressey]

Addresses #23 directly by adding a Befunge-93 cell signedness checker, as mentioned there.

I think this PR fixes (either the source or the description of) all the examples in #20 that fail to do what they say because they rely on unsigned char playfield cells. If there are any others where cell signedness is definitely the culprit, they could be added here, though I think in the other cases it is mainly due to other things.

(OK, actually the issues of numer.bf turned out to not be anything about signedness, but it got fixed here anyway.)

[j4james]

Note that this program (I'm referring to the sign test) only works correctly for implementations with 8 bit memory cells, which in the wild world of Befunge interpreters is far from guaranteed. Any implementation with a larger cell size will report as unsigned whether they are or not. One obvious group being all the Befunge-98 interpreters, but it's not uncommon for Befunge-93 implementations to use larger memory cells too.

[cpressey]

@j4james That's true, so I've rewritten the test program to take a different approach that maybe gets more to the point: the Befunge-93 programmer is interested in what the largest and smallest values they can use are, regardless of how it's implemented under the hood.

(Compatibility between Befunge-93 and Befunge-98 is known to be, um, "approximate", so if running any of these programs in a Befunge-98 interpreter results in hilarity ensuing, I'm fine with that.)