revealer: Prefer min and nat size

revealer: Prefer min and nat size

Assume that the fully expanded revealer will likely get an allocation
that matches the child's minimum or natural allocation, so we
special-case these two values.

So when - due to the precision loss - multiple sizes would match the
current allocation, we don't pick one at random, we prefer the min and
nat size.

The preference of nat size over min sie was decided after an IRC vote,
we don't actually have an idea what's more likely to happen in the real
world.
Should we ever get better data, we might want to switch.