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/tests/attestation_validation/mod.rs

@@ -0,0 +1,9 @@
+pub mod helpers;
+mod tests;
+
+use super::bls;
+use super::db;
+use super::ssz;
+use super::types;
+use super::hashing;
+use super::validation;