clippy: turn on manual_let_else lint

Ok, this one might be controversial :). Purely stylistic. Uses the new
`let...else` syntax. Saves a line of code.

Author: apoelstra

Cargo.toml

@@ -103,7 +103,7 @@ manual_assert = "warn"
 manual_instant_elapsed = "warn"
 manual_is_power_of_two = "warn"
 manual_is_variant_and = "warn"
-manual_let_else = "allow"
+manual_let_else = "warn"
 manual_ok_or = "warn"
 manual_string_new = "warn"
 many_single_char_names = "warn"

src/witness.rs

@@ -114,9 +114,8 @@ impl WitnessValues {
     /// witnesses will be pruned and which won't be pruned. This check skips unassigned witnesses.
     pub fn is_consistent(&self, witness_types: &WitnessTypes) -> Result<(), Error> {
         for name in self.0.keys() {
-            let declared_ty = match witness_types.get(name) {
-                Some(ty) => ty,
-                None => continue,
+            let Some(declared_ty) = witness_types.get(name) else {
+                continue;
             };
             let assigned_ty = self.0[name].ty();
             if assigned_ty != declared_ty {