Add Abs() method to sdk.Int

[hallazzang]

Summary

Add Abs() method to sdk.Int, just like sdk.Dec.

Problem Definition

Getting an absolute value from an integer type is very basic feature, but is missing.
Without this simple Abs() method, one should write:

abs := val
if abs.IsNegative() {
        abs = abs.Neg()
}

Proposal

Just add Abs() method to sdk.Int like this(following conventions in types/int.go):

func abs(i *big.Int) *big.Int { return new(big.Int).Abs(i) }

...

// Abs returns the absolute value of Int.
func (i Int) Abs() Int {
        return Int{abs(i.i)}
}

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[hallazzang]

I'll work on this feature and submit PR.