AxisAlignedBB builder

[dktapps]

#95 removed mutable methods for AxisAlignedBB.

However, many usages within PM Block classes made use of several chained BB methods together.

It might be worth introducing an AxisAlignedBBBuilder (or an alt name that doesn't have 3 Bs in a row) to avoid this.

Then we could have stuff like (e.g.):

Builder::one()
    ->squash(Axis::X, 7 / 16)
    ->squash(Axis::Z, 7 / 16)
    ->trim(Facing::UP, 1 / 8)
    ->build(); //returns immutable AxisAlignedBB

That being said, I'm not clear if this is actually necessary, performance-wise.