Re-export derives from serde

[dtolnay]

Fixes #797. @oli-obk I am still not settled on whether this should be on or off by default but I am leaning toward off.

[oli-obk]

that provide only handwritten impls

I'd say these are really rare. I think the usability of serde increases a lot if the serde crate reexports the derive macros by default.

[oli-obk]

I didn't know that worked (reexporting derives). That's awesome!

[dtolnay]

On crates.io, of the 438 crates that depend on serde >=0.9, 17% of them do not depend on serde_derive.

anterofit, app_units, arccstr, blob, bodyparser, bson, chrono, cleverbot_io, core-graphics, cryogen_prelude, cssparser, elastic, elastic_types_derive_internals, envy, euclid, eval, gelf, generic-array, geojson, gluon_language-server, hopper, hyper_serde, ipc-channel, iron-json-response, jfs, json-color, json_io, jsonrpc-lite, jsonrpc-macros, juniper, kubewatch, linked-hash-map, mage, mdbook, meval, microsalt, mysql, ndarray, objectid, ocean, offscreen_gl_context, plist, preferences, record-query, reqwest, rmp-serde, rmpv, rocket_contrib, ruma-identifiers, ruma-signatures, rust_decimal, serde-bench, serde-encrypted-value, serde_ignored, serde_json, serde-pickle, serde-protobuf, serde_test, serde-transcode, serde_urlencoded, serde_utils, serde-value, serde-xml-rs, serde_yaml, sharp_pencil, sidekiq, sidekiq-rs, spaceralk, string_cache, tera, toml, treeflection, treeflection_derive, uhttp_json_api, url_serde, uuid

Some broad categories are:

• Data formats (bson, serde_json, serde_yaml, toml)
• Only handwritten impls (chrono, linked-hash-map, uuid)
• Adapters (serde-encrypted-value, serde_ignored)
• General serde helpers (serde_utils, serde-value)
• Serde interop for specific libraries (bodyparser, hyper_serde, rocket_contrib, url_serde)
• Serde traits in the public API (hopper, jsonrpc-macros, kubewatch, mage, preferences)
• Standard library data structures only (cleverbot_io, geojson)

And lots more miscellaneous situations. I think this shows that depending on serde but not serde_derive is a fairly common and diverse use case.

For comparison, pretty much the same fraction of crates (19%) depend on serde >=0.9 but not serde_json so I don't think the number by itself justifies enabling the re-export by default. All three of serde and serde_derive and serde_json provide well-factored and distinguishable functionality so it is fair to expect users to depend on what they need.

I think this change is a last resort for the use case explained in the comment - preserving sanity around a serde cargo cfg. I do not intend to push this as the recommended way to import derives.