Add IsValid function to Coin

[colin-axner]

Summary

denomination validation is not done for Coin unless you use its constructor

Problem Definition

validateDenom is only called on Coin when using NewCoin or ParseCoin. A module that uses only Coin and not Coins can't appropriately validate the denomination without redundant code. A user could create a Coin without the constructor to bypass validation.

Proposal

Add an IsValid function to Coin that calls validateDenom as well as checking that the coin is > 0.

---

For Admin Use

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