Move shuffling to its own crate, update hash fn

Previously blake2s-256 was being used, now blake2b-512[:32] is being
used.

beacon_chain/utils/shuffling/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "shuffling"
+version = "0.1.0"
+authors = ["Paul Hauner <paul@paulhauner.com>"]
+
+[dependencies]
+hashing = { path = "../hashing" }