Declaring types for local variables

[denismerigoux]

Right now, local variables inside scope variable definitions (let x equals ... in ... syntax) have their types inferred. However, if you define a local variable whose type cannot be inferred completely from its value (like []), this throws the desugared typing in a state of confusion and may trigger internal typing error.

This issue proposes to fix this problem with two measures :

• when raising the internal error in desugared when there's not enough type information to disambiguate operators, the error message should inform the user that they need to type their local variables more explicitly to resolve ambiguity ;
• users should be able to optionally type local variables with the syntax let x content Foo equals ... in ....

[AltGr]

Related to #738 in that it could make this case more likely to happen