add `let` to frontend syntax

tests/Tests/Parser.idr

@@ -426,7 +426,13 @@ tests = "parser" :- [
           (App (V "z1" {}) (V "z2" {}) _) _),
     parseMatch term "let x = a in let y = b in z"
       `(Let (PQ One _, PV "x" {}, V "a" {})
-        (Let (PQ One _, PV "y" {}, V "b" {}) (V "z" {}) _) _)
+        (Let (PQ One _, PV "y" {}, V "b" {}) (V "z" {}) _) _),
+    parseMatch term "let x = y in z ∷ Z"
+      `(Let (PQ One _, PV "x" {}, V "y" {})
+          (Ann (V "z" {}) (V "Z" {}) _) _),
+    parseMatch term "let x = y in z₁ ≡ z₂ : Z"
+      `(Let (PQ One _, PV "x" {}, V "y" {})
+          (Eq (Unused _, V "Z" {}) (V "z₁" {}) (V "z₂" {}) _) _)
   ],
 
   "definitions" :-