Correct when the Processor starts using the first key

It waited for CONFIRMATIONS + 1 confirmations, instead of CONFIRMATIONS
confirmations.

Also adds a lib interface to access the coin traits and its constants.

processor/src/additional_key.rs

@@ -0,0 +1,14 @@
+use ciphersuite::Ciphersuite;
+
+use crate::coins::Coin;
+
+// Generate a static additional key for a given chain in a globally consistent manner
+// Doesn't consider the current group key to increase the simplicity of verifying Serai's status
+// Takes an index, k, to support protocols which use multiple secondary keys
+// Presumably a view key
+pub fn additional_key<C: Coin>(k: u64) -> <C::Curve as Ciphersuite>::F {
+  <C::Curve as Ciphersuite>::hash_to_F(
+    b"Serai DEX Additional Key",
+    &[C::ID.as_bytes(), &k.to_le_bytes()].concat(),
+  )
+}