Require the high-level sign traits instead of SignFn in bindings

Methods like `StaticInvoice::sign` have their own signing-function
traits, so there's no actual need to use `SignFn` directly, its
just a useful generic wrapper. Sadly, because it uses `AsRef`
bounds, which aren't really practical to map in bindings, we can't
really expose it directly in bindings. Because there's an
alternative and its tricky to expose, we simply mark it no-export
here.

Author: TheBlueMatt

lightning/src/offers/merkle.rs

@@ -94,6 +94,9 @@ pub enum SignError {
 }
 
 /// A function for signing a [`TaggedHash`].
+///
+/// This is not exported to bindings users as signing functions should just be used per-signed-type
+/// instead.
 pub trait SignFn<T: AsRef<TaggedHash>> {
 	/// Signs a [`TaggedHash`] computed over the merkle root of `message`'s TLV stream.
 	fn sign(&self, message: &T) -> Result<Signature, ()>;