Use an external for ReactDOMRE.Style.combine and remove Obj.magic

[bloodyowl]

No description provided.

[sgny]

I've been waiting for #437 to be merged for a while, as we already implemented something similar for reason-react-native.

That one uses the same definition for combine, but does not revise unsafeAddProp. There is also an unsafeAddStyle which allows something like

unsafeAddStyle(someStyle, {"someProp1": someValue1, "someProp2": someValue2});

which avoids multiple object copies when there are multiple props to add.

It also allows creating a style as below

unsafeAddStyle(make(), {"someProp1": someValue1, "someProp2": someValue2});

but %identity is always available to convert such objects for use with combine anyway.

[rickyvetter]

Hey do you mind rebasing? This looks great to me.