SPARK-1057 (alternative) Remove fastutil

(This is for discussion at this point -- I'm not suggesting this should be committed.)

This is what removing fastutil looks like. Much of it is straightforward, like using `java.io` buffered stream classes, and Guava for murmurhash3.

Uses of the `FastByteArrayOutputStream` were a little trickier. In only one case though do I think the change to use `java.io` actually entails an extra array copy.

The rest is using `OpenHashMap` and `OpenHashSet`.  These are now written in terms of more scala-like operations.

`OpenHashMap` is where I made three non-trivial changes to make it work, and they need review:

- It is no longer private
- The key must be a `ClassTag`
- Unless a lot of other code changes, the key type can't enforce being a supertype of `Null`

It all works and tests pass, and I think there is reason to believe it's OK from a speed perspective.

But what about those last changes?