Implement set repr

[corona10]

Expected

>>> a = {1, 2, 3}
>>> a
{1, 2, 3}

Actual

>>> a = {1, 2, 3}
>>> a
<set instance at 0xc0000aa018>

[xarus01]

Hi, mind if I try on this issue?

[corona10]

@xarus01 Please go ahead

[SanggiHong]

@xarus01 @corona10 May I take this issue?
I'd like to implement operations of the set, so it would be very helpful if this is implemented before the work.