Consolidate "FFI" and "FFI tips"

[soupi]

It have been asked a few times on IRC:

• "How do I return an Either String Int from JS?"
• "How do I return MyCustomDatatype from JS?"

Two solutions that were brought up were to:

• send the possible data constructors to be return (example, Left, Right, MyCustomDatatypeConstructor`, etc.),
• send a function of type String :: Maybe MyCustomDatatype that will get a string version of that constructor and will decide which value to return based on that.

It'd be great if that'd be documented in the FFI guide. :)

[garyb]

What, like this? 😄

[garyb]

Agreed it should be more prominent / perhaps expanded though.

[soupi]

Ah, sorry! I skimmed the document a few times and concluded that part wasn't documented but here it is...

[garyb]

It's under "FFI Tips" rather than just "FFI" in the guides, we should probably consolidate them :)

[chexxor]

Oh nice! I didn't know that was documented. We need to improve the organization of this repo.

[soupi]

oh, missed it again. Yeah it would definitely help to merge the two guides.

[hdgarrood]

There's also an FFI page under "language". I think we should try to put it all under the language one.