remove fastutil, add guava, add test-slf4j-simple

the fastutil addition should never have been merged without
addressing the enormous and almost entirely untouched mountain
of code that it pulls in. there are several possible solutions,
but I assert that the burden is on someone who cares more about
QDigest to implement one.

conversely, guava is tiny by comparison, has a variety of useful
classes, and is widely enough used that most downstream users will
see no increase in dependency count anyway.