remove the TranslatePk trait

We actually just empty it and deprecate it in an attempt to minimize
breakage. People *implementing* the trait will break anyway because the
next commit will have me changing the Translator trait. But the typical
case is that somebody imports the trait, then calls .translate_pk on
some object, and we want that usecase to continue working with only
warnings.

Author: apoelstra

src/descriptor/key.rs

@@ -1003,9 +1003,7 @@ impl DefiniteDescriptorKey {
     /// always return a compressed key
     ///
     /// Will return an error if the descriptor key has any hardened derivation steps in its path. To
-    /// avoid this error you should replace any such public keys first with [`translate_pk`].
-    ///
-    /// [`translate_pk`]: crate::TranslatePk::translate_pk
+    /// avoid this error you should replace any such public keys first with [`crate::Descriptor::translate_pk`].
     pub fn derive_public_key<C: Verification>(
         &self,
         secp: &Secp256k1<C>,

src/lib.rs

@@ -368,31 +368,14 @@ impl<E: fmt::Debug> fmt::Debug for TranslateErr<E> {
 
 /// Converts a descriptor using abstract keys to one using specific keys. Uses translator `t` to do
 /// the actual translation function calls.
+#[deprecated(since = "TBD", note = "This trait no longer needs to be imported.")]
 pub trait TranslatePk<P, Q>
 where
     P: MiniscriptKey,
     Q: MiniscriptKey,
 {
-    /// The associated output type. This must be `Self<Q>`.
-    type Output;
-
-    /// Translates a struct from one generic to another where the translations
-    /// for Pk are provided by the given [`Translator`].
-    fn translate_pk<T, E>(&self, translator: &mut T) -> Result<Self::Output, TranslateErr<E>>
-    where
-        T: Translator<P, Q, E>;
 }
 
-/// Either a key or keyhash, but both contain Pk
-// pub struct ForEach<'a, Pk: MiniscriptKey>(&'a Pk);
-
-// impl<'a, Pk: MiniscriptKey<Hash = Pk>> ForEach<'a, Pk> {
-//     /// Convenience method to avoid distinguishing between keys and hashes when these are the same type
-//     pub fn as_key(&self) -> &'a Pk {
-//         self.0
-//     }
-// }
-
 /// Trait describing the ability to iterate over every key
 pub trait ForEachKey<Pk: MiniscriptKey> {
     /// Run a predicate on every key in the descriptor, returning whether