Smash serai-client so the processors don't need the entire lib to access their specific code

We prior controlled this with feature flags. It's just better to define their
own crates.

tests/no-std/src/lib.rs

@@ -30,4 +30,7 @@ pub mod alloc {
   pub use frost_schnorrkel;
 
   pub use bitcoin_serai;
+
+  pub use serai_client_bitcoin;
+  pub use serai_client_ethereum;
 }