New syntax for typed literal arrays

Currenlty
~~~.nit
var a = [1, 2, 3] # a isa Array[Int]
var b = [1, true] # Type Error: ambiguous array type Int Bool
var c = new Array[Object].with_items(1, true) # c isa Array[Object]
~~~

Now
~~~.nit
var d = [1, true: Object] # d isa Array[Object]
var e = [1, 2: Numeric] # e isa Array[Numeric]
~~~