workarounds for supporting higher rank types in proud

[thautwarm]

use a special syntax, by introducing a soft keyword hrt:

hrt let x : forall a. (forall b. b -> a -> a) -> forall b. b -> a -> a = 
      fun f -> fun b -> fun a -> f b a

The hrt annotated declaration cannot contain scoped type variables.

However, without hrt keyword, we can use arbitrary (D)ML types, we can use a lot of nice to have features under syntax-directed generalisation rule, for free!