Add Jsx.addKeyProp function

[mununki]

This PR adds Jsx.addKeyProp in order to add the value of key into the props for React API. This function is needed to pass the type checking to the React component's props type which doesn't have key anymore. rescript-lang/syntax#635

// original
<Foo key="k" x="x" />

// converted
React.createElement(Foo.make, Jsx.addKeyProp(({x: "x"}: Foo.props<_>), "k")),
//                            ^^^^^^^^^^^^^^

[mununki]

Not sure whether [@@inline] is correct or not. I'm wondering [@@bs.inline] is current, but it fails to be compiled. Unused attribute ... error is thrown.

[mununki]

Js output seems incorrect either.

// generated
Jsx.addKeyProp({ x: "x" }, { key: "k1"})

// expected
Object.assign({ x: "x" }, { key: "k1"})

[cristianoc]

Same:

let is_inline : attr -> bool =
 fun ({ txt }, _) -> txt = "bs.inline" || txt = "inline"

I think it's more like inline is not so well specified. Not even sure it works across files.

[cristianoc]

I would assume a bundler would inline it anyway?

[mununki]

I would assume a bundler would inline it anyway?

Worth checking. The current output is working fine without any issues in the example project anyway.

[cristianoc]

OK to merge?