Added a function to let one spawn plants at mapgen time,
but it's slow as molasses in January.  Not recommended for
general use yet.
sped up the biome evaluation code somewhat, but it's still slow.
Documented the new mapgen-based spawn function.
tweaks to the documentation/comments for the mapgen function
just a minor logic tweak to try to speed the mapgen code up further.
sped up mapgen functions massively.  I think it's suitable
for general use now!