Follow conventional feature-gating for serde

[lloydmeta]

Closes #97

• Follow guidelines in https://rust-lang-nursery.github.io/api-guidelines/interoperability.html#data-structures-implement-serdes-serialize-deserialize-c-serde
• Add serde derivation for Coproducts as well

[ExpHP]

frunk should also do this, not just core.

But hmmm... I dont think it will be possible to have the serde feature of frunk toggle the serde feature of core when using this hack. Let me think about this...

[ExpHP]

Here's how num does it:

https://github.com/rust-num/num/blob/master/Cargo.toml

Basically, num itself doesn't contain anything, so it doesn't need serde as a dependency. That allows "serde" to be used as a feature name.

For frunk to be able to do that, everything in frunk that doesn't come from frunk-core would need to be split out into a third crate. That's kind of a big change! :P

Still looking for a better alternative...

[ExpHP]

Okay, so it looks like having the features be disconnected from each other is already the status quo for frunk, so this is my recommendation for now:

• Make the same changes to frunk's Cargo.toml.
• Add a note to README.md that enabling serde requires one to explicitly depend on frunk-core: (note: this was already true before this PR, but I think it is worth documenting now)

[dependencies]
frunk = { version = "0.1.36", features = ["serde"] }
frunk-core = { version = "0.1.36", features = ["serde"] }

[lloydmeta]

@ExpHP Aaah right; good point. I've made the same change to frunk and updated the readme. Let me know what you think.

[ExpHP]

Hm, that's funny. I didn't realize this earlier, but: judging from how the second commit doesn't touch any .rs files, I guess there isn't actually anything in frunk itself that has serde derives? :stuck_out_tongue:

Okay, so technically you can do what num does, and add a serde feature to frunk that activates the feature "frunk-core/serde". The advantage of this would be that a user doesn't need to depend on frunk-core explicitly.

Or: what you just commited is also fine! Even if it does seem a bit silly at the moment, it has the advantage that (non-core) frunk will be able to backwards-compatibly add things that use serde in the future.

(if you go with option 2 then LGTM)

[lloydmeta]

Yeah, frunk doesn't at the moment, just thought it might be nice to have something there in case it did in the future. I think I'll go with option 2 for the added backwards-compatibility.

Thanks for the review !