emit IR for index access for array

but multi-dimention array is not available yet because nested array
type IR cannot be emit.

func main
    var i := 1
    var a := [1, 2, 3]
    println(a[1])
    println(a[i])

    a2 := [1, 2, 3]
    println(a2[1])
    println(a2[i])

    var j := 2

    # 2 dimension array can not be emit yet
    # because the length of the type's array
    # is not specified.

    # var a3 := [[1], [2], [-1, -2, -3]]
    # println(a3[2][1])
    # println(a3[2][i])
    # println(a3[j][1])
    # println(a3[j][i])

    # a4 := [[1], [2], [-1, -2, -3]]
    # println(a4[2][1])
    # println(a4[2][i])
    # println(a4[j][1])
    # println(a4[j][i])

end