Constant babalcore::WIDTH_MAX

pub const WIDTH_MAX: usize = 32;

A maximum width, most playable values should range from 6 to 12, so 32 is already a lot. To simplify/optimize code, program uses fixed size rows, set to the WIDTH_MAX. With slabs of 4 or 8 bytes, it means only 256 bytes (worst case) for a row. With this setting, a 10_000 rows level, playable for about an hour, at 3 slabs/sec still fits into 30Mb of memory.