Support other branching factors than 16

[vorner]

It would be nice if a user could customize the branching factor.

Currently, the branching factor is configured by constants at the top of raw/mod.rs (LEVEL_BITS, LEVEL_MASK, LEVEL_CELLS, MAX_LEVELS).

The first step here would be moving the constants into the Config trait so it can be overridden. To protect from inconsistent values, add the tests from consts_consistent as asserts to the Raw::with_hasher constructor.

For now I believe this would be uncommon to customize, so it can probably stay on Raw only.

If anyone wants to work on this, please claim it by commenting to avoid work duplication. In case of any questions or need for help, don't hesitate to ask.