Add pre-written validation code

This adds block and attestation validation code that was written
previously. There were many non-validation specific changes made whilst
building these functions (e.g., db, hashing, etc) -- these changes have
already been merged into master and this branch has been created just to
make it easy to review this code.

beacon_chain/validation/src/lib.rs

@@ -0,0 +1,12 @@
+extern crate db;
+extern crate bls;
+extern crate hashing;
+extern crate ssz;
+extern crate ssz_helpers;
+extern crate types;
+
+pub mod attestation_validation;
+mod attestation_parent_hashes;
+pub mod block_validation;
+mod message_generation;
+mod signature_verification;