Implement polymorphic let bindings

[maclement]

Description

Add type arguments to let bindings as shown below.

let l @a = length @a in l @Integer [1,2,3] + l @Bool [True,False]

Motivation

The current implementation does not infer the type of polymorphic let bindings correctly. These polymorphic let bindings are allowed in Haskell and are probably needed for the translation of guards. The following is allowed in Haskell for example.

let l = length in l [1,2,3] + l [True,False]

Tasks

• Add type arguments to the Bind type.
• Adjust TypeInferencePass for let expressions