Work on fleshing out and profiling soc's BigInt type

[non]

Simon Ochsenreither has done significant work on implementing a from-scratch BigInt type, using some patches from Tim Buktu and other Java numerical folks. This task is about fleshing out those types, benchmarking them (to make sure they represent an improvement over java.math.BigInteger) and then plugging them into Spire.

This obviates the need for BigInt (which is obviously a wrapper around BigInteger), and might also obviate the need for SafeLong if we can build the same kind of fast-path for Long values into it.

[denisrosset]

Regrouped.

[armanbilge]

I think some of these ideas found their way into scala/scala#9628.