The very core of cross() is capable of suffering integer overflow on

The very core of cross() is capable of suffering integer overflow on
large puzzles. Resort to hand-hacked 64-bit arithmetic for doing dot
products; everything else remains in `long' for the moment.

(Ideally I'd auto-detect the presence of `long long' and use it in
place of my cheap plastic imitation where possible, but since I
currently don't have a configure mechanism that'll have to wait.)

[originally from svn r6137]