[CameLIGO] Fix type variable binding in expressions

Motivation and Context

There was a bug when there were type variable bindings in both LHS & RHS,

e.g. For

let foo (type a) : a list -> a list = 
   fun (type b) (xs : b list) : b list -> xs

The abstraction was

const foo : ∀ a : * . list (a) -> list (a) =
  Λ a ->  fun ( xs : list (b)) : list (b) -> xs

There was no type abstraction for b

Fixes #1669 (closed)

Description

This MR fixes the issue

const foo : ∀ a : * . list (a) -> list (a) =
  Λ a ->  Λ b ->  fun ( xs : list (b)) : list (b) -> xs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

For a cameligo file like

let foo (type a) : a list -> a list = 
   let id = fun (type b) (xs : b list) : b list -> xs in
   fun (xs : a list) : a list -> id xs 

let main (_ : unit * int list) : operation list * int list =
  [], foo [1 ; 2 ; 3]

Before

$ ligo.60 compile contract x.mligo 
File "x.mligo", line 2, characters 31-32:
  1 | let foo (type a) : a list -> a list = 
  2 |    let id = fun (type b) (xs : b list) : b list -> xs in
  3 |    fun (xs : a list) : a list -> id xs 

Type "b" not found. 

After

$ ligo compile contract x.mligo 
{ parameter unit ;
  storage (list int) ;
  code { DROP ;
         NIL int ;
         PUSH int 3 ;
         CONS ;
         PUSH int 2 ;
         CONS ;
         PUSH int 1 ;
         CONS ;
         NIL operation ;
         PAIR } }

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).