Foralls do not typecheck

[yannham]

[EDIT] I misunderstood something and jumped to conclusions. Polymorphic instantiation works (almost, see #88) fine

Describe the bug
Valid programs annotated with polymorphic types, that is with a Promise(ty, foo) where ty contains a forall, do not typecheck.

For example, executing (with some preparation first, see To reproduce) src/examples/forall.ncl containing

let f = Promise(forall a. (forall b. a -> b ->  a), fun x => fun y => x) in
f

with nickel <src/examples/forall.ncl results in the error message:

Typechecked: Err("The following types dont match Arrow(Constant(1), Concrete(Arrow(Constant(2), Constant(1)))) 
 -- Forall(Ident(\"a\"), Concrete(Forall(Ident(\"b\"), Concrete(Arrow(Concrete(Var(Ident(\"a\"))), Concrete(Arrow(Concrete(Var(Ident(\"b\"))), Concrete(Var(Ident(\"a\"))))))))))")
[...]

To Reproduce
This is not directly visible as there is a strict parameter, currently set to false when the typechecker starts, which basically disable typechecking if not set. It corresponds to "dynamic typing" mode, but it should still enforce the type of Promise: why it does not is a different matter. A quick way to isolate this issue is set it to true from the beginning, but then automatically included contracts do not typecheck anymore:

• set strict to true in typecheck.rs
• disable the automatic inclusion of the basic contracts by setting include_contracts to false in program.rs
• cargo build
• target/debug/nickel <src/examples/forall.ncl

Expected behavior
The program should be accepted by the typechecker:

Typechecked: Ok(Types(Arrow(Types(Dyn), Types(Arrow(Types(Dyn), Types(Dyn))))))
[...]