Add a Scanner to bitcoin-serai

Moves the processor to it. This ends up as a net-neutral LoC change to the
processor, unfortunately, yet this makes bitcoin-serai safer/easier to use, and
increases the processor's usage of bitcoin-serai.

Also re-organizes bitcoin-serai a bit.

coins/bitcoin/src/tests/mod.rs

@@ -14,8 +14,7 @@ use frost::{
 };
 
 use crate::{
-  crypto::{x_only, make_even},
-  algorithm::Schnorr,
+  crypto::{x_only, make_even, Schnorr},
   rpc::Rpc,
 };