Merge spec into types crate.

Also remove some pre-2018 `extern crate` calls.

eth2/validator_induction/src/inductor.rs

@@ -1,6 +1,5 @@
 use bls::verify_proof_of_possession;
-use spec::ChainSpec;
-use types::{BeaconState, Deposit, Validator};
+use types::{BeaconState, ChainSpec, Deposit, Validator};
 
 #[derive(Debug, PartialEq, Clone)]
 pub enum ValidatorInductionError {

eth2/validator_induction/src/lib.rs

@@ -1,8 +1,3 @@
-extern crate bls;
-extern crate hashing;
-extern crate spec;
-extern crate types;
-
 mod inductor;
 
 pub use crate::inductor::{process_deposit, ValidatorInductionError};