Make behavior between curly- and paren-delimited blocks consistent.

Note that the formatter rewrite parens-block sequences as curly-block
  sequences anyway. Albeit weird looking syntax, they are valid
  nonetheless.

  I also clarified a bit the hints and description of the
  'illegal::return' error as it would mistakenly say 'function' instead
  of 'block'.

crates/aiken-lang/src/parser/error.rs

@@ -173,10 +173,10 @@ pub enum ErrorKind {
         hint: Option<String>,
     },
 
-    #[error("I discovered an unfinished assignment.")]
+    #[error("I spotted an unfinished assignment.")]
     #[diagnostic(
         help(
-            "{} and {} bindings must be followed by a valid expression.",
+            "{} and {} bindings must be followed by a valid, complete, expression.",
             "let".if_supports_color(Stdout, |s| s.yellow()),
             "expect".if_supports_color(Stdout, |s| s.yellow()),
         ),

crates/aiken-lang/src/parser/expr/block.rs

@@ -11,22 +11,22 @@ pub fn parser(
     choice((
         sequence
             .clone()
-            .delimited_by(just(Token::LeftBrace), just(Token::RightBrace))
-            .map_with_span(|e, span| {
-                if matches!(e, UntypedExpr::Assignment { .. }) {
-                    UntypedExpr::Sequence {
-                        location: span,
-                        expressions: vec![e],
-                    }
-                } else {
-                    e
-                }
-            }),
+            .delimited_by(just(Token::LeftBrace), just(Token::RightBrace)),
         sequence.clone().delimited_by(
             choice((just(Token::LeftParen), just(Token::NewLineLeftParen))),
             just(Token::RightParen),
         ),
     ))
+    .map_with_span(|e, span| {
+        if matches!(e, UntypedExpr::Assignment { .. }) {
+            UntypedExpr::Sequence {
+                location: span,
+                expressions: vec![e],
+            }
+        } else {
+            e
+        }
+    })
 }
 
 #[cfg(test)]

crates/aiken-lang/src/parser/expr/snapshots/expect_let_in_let_parens.snap

@@ -1,15 +1,34 @@
 ---
 source: crates/aiken-lang/src/parser/expr/assignment.rs
-description: "Invalid code (parse error):\n\nlet a = ( let b = 42 )"
+description: "Code:\n\nlet a = ( let b = 42 )"
 ---
-[
-    ParseError {
-        kind: UnfinishedAssignmentRightHandSide,
-        span: 0..22,
-        while_parsing: None,
-        expected: {},
-        label: Some(
-            "invalid assignment right-hand side",
-        ),
+Assignment {
+    location: 0..22,
+    value: Sequence {
+        location: 8..22,
+        expressions: [
+            Assignment {
+                location: 10..20,
+                value: UInt {
+                    location: 18..20,
+                    value: "42",
+                    base: Decimal {
+                        numeric_underscore: false,
+                    },
+                },
+                pattern: Var {
+                    location: 14..15,
+                    name: "b",
+                },
+                kind: Let,
+                annotation: None,
+            },
+        ],
     },
-]
+    pattern: Var {
+        location: 4..5,
+        name: "a",
+    },
+    kind: Let,
+    annotation: None,
+}