Configurable width

[rvagg]

The width provides the primary tuneable parameter, allowing for a selection along the block-size, mutation cost and traversal cost spectrums. A fixed width of 8 provides for a maximum arity of 8 which. It's also something that probably should be benchmarked to understand the true costs involved but we can't do that without the ability to change it. Some Bmap code in here suggests that variability was a TODO, but a single byte is the easy case and that's taken advantage of here, hence the 8.

[Stebalien]

Fixed.