miniscript: make dissat_sat return a struct rather than a tuple

It is too easy to get the satisfactions and dissatisfactions confused. Use
a struct with named fields.

Curiously, outside of this module dissatisfactions are never used, so we can
keep the `dissat` field private. (Well, this is a bit of a sketchy thing to
say -- dissatisfactions **are** used by the Satisfaction::thresh and
thresh_mall functions, but those functions take dissats and sats as independent
Vecs they can manipulate, rather than taking a SatDissat struct.)

Author: apoelstra

src/miniscript/satisfy/mod.rs

@@ -1026,7 +1026,7 @@ impl<Pk: MiniscriptKey + ToPublicKey> Satisfaction<Placeholder<Pk>> {
         Ctx: ScriptContext,
         P: AssetProvider<Pk>,
     {
-        Self::dissat_sat(node, provider, false, root_has_sig, leaf_hash).1
+        Self::dissat_sat(node, provider, false, root_has_sig, leaf_hash).sat
     }
 
     pub(crate) fn build_template_mall<P, Ctx>(
@@ -1039,7 +1039,7 @@ impl<Pk: MiniscriptKey + ToPublicKey> Satisfaction<Placeholder<Pk>> {
         Ctx: ScriptContext,
         P: AssetProvider<Pk>,
     {
-        Self::dissat_sat(node, provider, true, root_has_sig, leaf_hash).1
+        Self::dissat_sat(node, provider, true, root_has_sig, leaf_hash).sat
     }
 
     // produce a non-malleable satisafaction for thesh frag