Use BTreeSet internally

The `BTreeSet` is a drop in replacement for the `HashSet` with the
benefit that it is available in no-std environments.

We have a few uses of `HashSet`s internally within functions, we can use
`BTreeSet` instead.

Done in preparation for removing the `hashbrown` dependency.

Author: tcharding

src/miniscript/analyzable.rs

@@ -213,7 +213,7 @@ impl<Pk: MiniscriptKey, Ctx: ScriptContext> Miniscript<Pk, Ctx> {
         // to have an iterator
         let all_pkhs_len = self.iter_pk().count();
 
-        let unique_pkhs_len = self.iter_pk().collect::<HashSet<_>>().len();
+        let unique_pkhs_len = self.iter_pk().collect::<BTreeSet<_>>().len();
 
         unique_pkhs_len != all_pkhs_len
     }

src/policy/concrete.rs

@@ -791,7 +791,7 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
     pub fn check_duplicate_keys(&self) -> Result<(), PolicyError> {
         let pks = self.keys();
         let pks_len = pks.len();
-        let unique_pks_len = pks.into_iter().collect::<HashSet<_>>().len();
+        let unique_pks_len = pks.into_iter().collect::<BTreeSet<_>>().len();
 
         if pks_len > unique_pks_len {
             Err(PolicyError::DuplicatePubKeys)