Fix forwardRef unsafety + make DOM refs types more idiomatic

[jchavarri]

Fixes #13.

This PR:

• Fixes Ref.t functions current and setCurrent which were pointing to nonexistent ReactJS functions
• Makes DOM refs types more idiomatic by being option, not Js.opt
• Defines forwardRef ref to be of type ReactDOM.domRef to avoid unsafety issues
• Updates the ppx so forwardRef usages get the right signature for ref including ReactDOM.domRef
• Moves forwardRef to ReactDOM module to avoid circular dependencies problems
• Adds a new ppx test for forwardRef processing