Continue filling out main loop

Adds generics to the db_channel macro, fixes the bug where it needed at least
one key.

processor/bitcoin/src/primitives/mod.rs

@@ -1,3 +1,20 @@
+use ciphersuite::{Ciphersuite, Secp256k1};
+
+use bitcoin_serai::bitcoin::key::{Parity, XOnlyPublicKey};
+
 pub(crate) mod output;
 pub(crate) mod transaction;
 pub(crate) mod block;
+
+pub(crate) fn x_coord_to_even_point(key: &[u8]) -> Option<<Secp256k1 as Ciphersuite>::G> {
+  if key.len() != 32 {
+    None?
+  };
+
+  // Read the x-only public key
+  let key = XOnlyPublicKey::from_slice(key).ok()?;
+  // Convert to a full public key
+  let key = key.public_key(Parity::Even);
+  // Convert to k256 (from libsecp256k1)
+  Secp256k1::read_G(&mut key.serialize().as_slice()).ok()
+}