build: embedding chipmunk 6.2.1

because it will hopefully make cross-compilation and embedding of
chipmunk easier. with a working cross-chicken, you should be able to do

  cross-chicken-install chickmunk

and you've got the egg, without the need to port chickmunk to
android/ios as well - the chicken cross toolchain might be able to do
this automatically for us.
refac major: renames chickmunk => acorn

because chickmunk is a too confusing name. chipmunks love acorns so
that's what we're going for now.
build cleanup: removes Android.mk file

because we don't really need this anymore. this was part of an old and
messy build-system. a better idea is to use a cross-chicken for building
against Android.
doc: updates readme for Acorn