Revise desugaring following feedback

- We now consistently desugar an expect in the last position as
    `Void`. Regardless of the pattern. Desugaring to a boolean value is
    deemed too confusing.

  - This commit also removes the desugaring for let-binding. It's only
    ever allowed for _expect_ which then behaves like a side effect.

  - We also now allow tests to return either `Bool` or `Void`. A test
    that returns `Void` is treated the same as a test returning `True`.

crates/uplc/src/machine/eval_result.rs

@@ -39,7 +39,11 @@ impl EvalResult {
         } else {
             self.result.is_err()
                 || matches!(self.result, Ok(Term::Error))
-                || !matches!(self.result, Ok(Term::Constant(ref con)) if matches!(con.as_ref(), Constant::Bool(true)))
+                || !matches!(
+                  self.result,
+                  Ok(Term::Constant(ref con))
+                  if matches!(con.as_ref(), Constant::Bool(true)) || matches!(con.as_ref(), Constant::Unit)
+                )
         }
     }