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/benches/main.rs

@@ -0,0 +1,5 @@
+#![feature(test)]
+extern crate lighthouse;
+extern crate tests;
+
+mod block_validation;