Substrate relay stub (#376)

* substrate-relay: initial commit

* MillauHeaderApi and RialtoHeaderApi

* post-merge fixes + TODOs + compilation

bridges/bin/millau-runtime/src/lib.rs

@@ -61,7 +61,7 @@ pub use sp_runtime::BuildStorage;
 pub use sp_runtime::{Perbill, Permill};
 
 /// An index to a block.
-pub type BlockNumber = u32;
+pub type BlockNumber = bp_millau::BlockNumber;
 
 /// Alias to 512-bit hash when used in the context of a transaction signature on the chain.
 pub type Signature = MultiSignature;
@@ -81,7 +81,7 @@ pub type Balance = u128;
 pub type Index = u32;
 
 /// A hash of some data used by the chain.
-pub type Hash = sp_core::H256;
+pub type Hash = bp_millau::Hash;
 
 /// Digest item type.
 pub type DigestItem = generic::DigestItem<Hash>;