Test bitcoin-serai

Also resolves a few rough edges.

coins/bitcoin/src/lib.rs

@@ -2,11 +2,12 @@
 pub use bitcoin;
 
 /// Cryptographic helpers.
+#[cfg(feature = "hazmat")]
 pub mod crypto;
+#[cfg(not(feature = "hazmat"))]
+pub(crate) mod crypto;
+
 /// Wallet functionality to create transactions.
 pub mod wallet;
 /// A minimal asynchronous Bitcoin RPC client.
 pub mod rpc;
-
-#[cfg(test)]
-mod tests;