super_errors_multi: cover typecore Modules_not_allowed and Unexpected_existential

Author: JonoPrest

tests/build_tests/super_errors_multi/expected/Modules_not_allowed_toplevel.expected

@@ -0,0 +1,11 @@
+===== User.res =====
+
+  We've found a bug for you!
+  /.../fixtures/Modules_not_allowed_toplevel/User.res:8:5-13
+
+  6 │   )
+  7 │ 
+  8 │ let module(M) = make()
+  9 │ 
+
+  Modules are not allowed in this pattern.

tests/build_tests/super_errors_multi/expected/Unexpected_existential_in_let.expected

@@ -0,0 +1,11 @@
+===== User.res =====
+
+  We've found a bug for you!
+  /.../fixtures/Unexpected_existential_in_let/User.res:3:5-17
+
+  1 │ let value: Gadt.wrap<unit> = Gadt.Pack(1)
+  2 │ 
+  3 │ let Gadt.Pack(_x) = value
+  4 │ 
+
+  Unexpected existential

tests/build_tests/super_errors_multi/fixtures/Modules_not_allowed_toplevel/Sig.res

@@ -0,0 +1,3 @@
+module type S = {
+  let v: int
+}

tests/build_tests/super_errors_multi/fixtures/Modules_not_allowed_toplevel/User.res

@@ -0,0 +1,8 @@
+let make = (): module(Sig.S) =>
+  module(
+    {
+      let v = 1
+    }
+  )
+
+let module(M) = make()

tests/build_tests/super_errors_multi/fixtures/Unexpected_existential_in_let/Gadt.res

@@ -0,0 +1 @@
+type rec wrap<_> = Pack('a): wrap<unit>

tests/build_tests/super_errors_multi/fixtures/Unexpected_existential_in_let/User.res

@@ -0,0 +1,3 @@
+let value: Gadt.wrap<unit> = Gadt.Pack(1)
+
+let Gadt.Pack(_x) = value