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/aiken-lang/src/parser/definition/snapshots/def_invalid_property_test.snap

@@ -52,14 +52,7 @@ Test(
         location: 0..26,
         name: "foo",
         public: false,
-        return_annotation: Some(
-            Constructor {
-                location: 0..39,
-                module: None,
-                name: "Bool",
-                arguments: [],
-            },
-        ),
+        return_annotation: None,
         return_type: (),
         end_position: 38,
         on_test_failure: FailImmediately,

crates/aiken-lang/src/parser/definition/snapshots/def_property_test.snap

@@ -37,14 +37,7 @@ Test(
         location: 0..28,
         name: "foo",
         public: false,
-        return_annotation: Some(
-            Constructor {
-                location: 0..41,
-                module: None,
-                name: "Bool",
-                arguments: [],
-            },
-        ),
+        return_annotation: None,
         return_type: (),
         end_position: 40,
         on_test_failure: FailImmediately,

crates/aiken-lang/src/parser/definition/snapshots/def_property_test_annotated_fuzzer.snap

@@ -44,14 +44,7 @@ Test(
         location: 0..26,
         name: "foo",
         public: false,
-        return_annotation: Some(
-            Constructor {
-                location: 0..39,
-                module: None,
-                name: "Bool",
-                arguments: [],
-            },
-        ),
+        return_annotation: None,
         return_type: (),
         end_position: 38,
         on_test_failure: FailImmediately,

crates/aiken-lang/src/parser/definition/snapshots/def_test.snap

@@ -13,14 +13,7 @@ Test(
         location: 0..10,
         name: "foo",
         public: false,
-        return_annotation: Some(
-            Constructor {
-                location: 0..23,
-                module: None,
-                name: "Bool",
-                arguments: [],
-            },
-        ),
+        return_annotation: None,
         return_type: (),
         end_position: 22,
         on_test_failure: FailImmediately,

crates/aiken-lang/src/parser/definition/snapshots/def_test_fail.snap

@@ -44,14 +44,7 @@ Test(
         location: 0..26,
         name: "invalid_inputs",
         public: false,
-        return_annotation: Some(
-            Constructor {
-                location: 0..61,
-                module: None,
-                name: "Bool",
-                arguments: [],
-            },
-        ),
+        return_annotation: None,
         return_type: (),
         end_position: 60,
         on_test_failure: SucceedEventually,

crates/aiken-lang/src/parser/definition/test.rs

@@ -45,7 +45,7 @@ pub fn parser() -> impl Parser<Token, ast::UntypedDefinition, Error = ParseError
                 end_position: span.end - 1,
                 name,
                 public: false,
-                return_annotation: Some(ast::Annotation::boolean(span)),
+                return_annotation: None,
                 return_type: (),
                 on_test_failure: fail.unwrap_or(OnTestFailure::FailImmediately),
             })