feat(franklin-crypto): add extern crate core; to lib.rs (#27)

I wasn't able to quickly switch to newer edition Rust in this repo. 
This statement in `lib.rs` fixes my dependency issues on era side. 

Please LMK if there are any reprecussions of this.

crates/franklin-crypto/src/lib.rs

@@ -8,6 +8,7 @@ extern crate blake2;
 extern crate blake2_rfc_bellman_edition as blake2_rfc;
 pub extern crate boojum;
 extern crate byteorder;
+extern crate core;
 extern crate derivative;
 extern crate digest;
 extern crate indexmap;