Move shuffling functions around

- Move `delegation` from the `transition` dir into its own dir:
`beacon_chain/validator_shuffling`
- Rename `beacon_chain/utils/shuffling` -> `vec_shuffle`

beacon_chain/utils/vec_shuffle/Cargo.toml

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