The typechecker rejects polymorphic annotations inside a Promise

[yannham]

Describe the bug
A program which contains a let annotated (using either Promise or Assume) by a polymorphic type, itself located inside a Promise (which triggers the "strict" or "static" typechecking mode), is unduly rejected.

To Reproduce

let f =
    Promise(forall a. a -> a, let g = 
    Promise(forall a. a -> a, fun x => x)
    in g)
in f 2

which results in

Typechecked: Err("The following types dont match Arrow(Constant(119), Constant(119))
-- Forall(Ident(\"a\"), Concrete(Arrow(Concrete(Var(Ident(\"a\"))), Concrete(Var(Ident(\"a\"))))))")
Evaluation finished with result:
Num(2.0)

Expected behavior
Well-typed programs with polymorphic annotations inside a promise should typecheck. In particular, the previous example should result in the output:

Typechecked: Ok(Types(Dyn))
Evaluation finished with result:
Num(2.0)