Infer more attribute types

[xymus]

Extend the detection of the static type of attributes from their literal values to support three new cases:

• Simple arrays like [0, 1, 2] and [new Set[Int], new Set[Int]]. However, it does not accept arrays with an explicit type because we can't subtype/anchor at that point, as far as I know.

• Negative integers and floats. This cheats a bit as the return type of the unary - is defined in the core libary. However this should help 99.9% of the time, in particular for Nit beginners, and a workaround is to declare the attribute static type when defining a different kernel library.

• The once keyword.

class A
       # Now detected
       var i = -1
       var f = -1.0
       var a = [0, 1]
       var o = once [0, 1]

       # These are refused
       var a1 = [0, 1.0, "a"] # Different types
       var a2 = [0, 1: Int] # Can't reliably check subtypes
       var a4 = [1+1] # Expression
       var o1 = once [0, "a"] # Forwarded error
end

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You may want to review commit by commit as the first commit is a small refactoring.