This is useful when we want to perform an
action on a commonly-occuring node (like
sand or item stacks) with a requirement of
a neighbor of a less common node type (like
igniters). Checking for B with A nearby can
be a lot faster than A with B nearby if B is
less common, so fewer neighbor checks
actually need to happen.
This was already done "manually" for the
flammable/igniter interaction, but now this is
a general API for all ABMs, that allows the
inversion to be done nearly transparently
with just a flag in the definition, and so the
ABM rules can be written more simply, with
the inversion handled elsewhere.
TODO: Rewrite current manual-inversions,
including the ones for flammable/igniter and
lava/stone hardening/softening.
7fd51c1a6c