rustdoc changes (source links and feature labels)

[qrnch-jan]

There are two proposed changes in this pull request:

• Make the docs generated on docs.rs use --generate-link-to-definitions, so the source is a little more convenient to brows on docs.rs. (See example here)
• Enable all features when generating docs on docs.rs and instead use feature labels in order to mark what features are required for individual items.

To generate docs with feature labels locally use:

$ RUSTFLAGS="--cfg docsrs" RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features

(cargo doc still works as usual).

[avastmick]

Happy. I will merge and release.

Thanks for the contribution! 😄