Update React components to use `React.forwardRef()`

[joshblack]

Currently our Icon components are exported as plain functional components. It would be great if we included support for forwarding refs through React.forwardRef(), however this is only available past React v16.3. Currently, the project lists React v15 as a peer dependency.

It seems like we have a couple of paths forward:

• Increase the peer dependency range, but still include v15, and try and feature detect forwardRef. This would land in a minor change to the package
• Increase the peer dependency range explicitly to >=v16.3 and unconditionally use forwardRef. This would land in a major change to the package

[lesliecdubs]

Open question that came up at PRC maintainer's sync: is there any issue with releasing this major change we would need to account for?

@colebemis has the most context on this repo and said it's a pretty safe breaking change to go ahead and make, but feel free to coordinate directly as needed.