Support no-std builds of bitcoin-serai

Arguably not meaningful, as it adds the scanner yet not the RPC, and no signing
code since modular-frost doesn't support no-std yet. It's a step in the right
direction though.

coins/bitcoin/src/wallet/send.rs

@@ -1,4 +1,4 @@
-use std::{
+use std_shims::{
   io::{self, Read},
   collections::HashMap,
 };