Box: avoid recalc/reassignments, improve var names

Box: avoid recalc/reassignments, improve var names

Homogeneous branches repeated the calculation/assignment of the initial
space available to children. This avoids that by shuffling some code.

Perhaps more importantly, in doing that, I ended up with some ambiguous
names, and Company and I realised how vague the pre-existing naming was.
"size" becomes "extra_space", as this is what it represents. Conversely,
"extra" becomes "size_given_to_child" (albeit still given out in two
different ways depending on whether the Box is homogeneous). My hope is
that these sections of code are now somewhat less baffling than before!