Add GoString method for Decimal and NullDecimal (#270)

[a69]

This PR is for support fmt.GoStringer in #270 issue

[treuherz]

I'd find a GoStringer implementation really helpful, for same reasons you describe in #270, but I think it would be more helpful for the method to print fmt.Sprintf("decimal.New(%d, %d)", d.value, d.exp) instead of RequireFromString. This has the advantage of using the most "natural" constructor in the package, and also will actually get you a string that lets you construct a Decimal with the same behaviour.

For example, under this implementation, you could have

d1 := decimal.New(1, 2)
d2 := decimal.New(100, 0)

println(d1.GoString()) // decimal.RequireFromString("100")
println(d2.GoString()) // decimal.RequireFromString("100")
println(d1.Exponent()) // 2
println(d2.Exponent()) // 0

I realise decimal.New(1, 2) is a little bit harder to read an intuitive value from than decimal.RequireFromString("100") but I think it's worth it for more consistent behaviour.

[treuherz]

Another possible wrinkle here: the user may not have imported the package as decimal. I think the options here are either to leave it, as it'll probably be fine in 99.99% of cases, or remove the decimal. from the method, which could be more confusing but at least won't be contradictory. The fmt package doesn't give explicit guidance here, although their example doesn't include the name.

[a69]

@treuherz
decimal.New works if d.value always fits in int64.
I think as it says in code comment decimal.go#L5:
// The best way to create a new Decimal is to use decimal.NewFromString

To fix this issue we could do something like:
s := fmt.Sprintf(`decimal.RequireFromString("%s")`, d.string(false)) if d.exp > 0 { s = s + fmt.Sprintf(".Round(%d)", -d.exp) } return s

[mwoss]

I've already responded in the linked issue. I think we might try initialization via NewBigInt to not to lose precision.