Move additional_key.rs to serai-processor-view-keys

I don't love this. I wanted to simply add this function to `processor/key-gen`,
but then anyone who wants a view key needs to pull in Bulletproofs which is a
mess of code. They'd also be subject to an AGPL licensed library.

This is so small it should be a primitive elsewhere, yet there is no primitives
library eligible. Maybe serai-client since that has the code to make
transactions to Serai (and will have this as a dependency)? Except then the
processor has to import serai-client when this rewrite removed it as a
dependency.

processor/view-keys/src/lib.rs

@@ -0,0 +1,13 @@
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![doc = include_str!("../README.md")]
+#![deny(missing_docs)]
+
+use ciphersuite::Ciphersuite;
+
+/// Generate a view key for usage within Serai.
+///
+/// `k` is the index of the key to generate (enabling generating multiple view keys within a
+/// single context).
+pub fn view_key<C: Ciphersuite>(k: u64) -> C::F {
+  C::hash_to_F(b"Serai DEX View Key", &k.to_le_bytes())
+}