Auto merge of #5902 - little-arhat:feature-add-required-features-to-metadata-output, r=alexcrichton
Auto merge of #5902 - little-arhat:feature-add-required-features-to-metadata-output, r=alexcrichton

Add `required-features` to `SerializedTarget`

This will add it to `cargo metadata` output and will make it
possible to enable features needed to build specific target.

tshepang/cargo,eminence/cargo,rust-lang/cargo,tshepang/cargo,rust-lang/cargo,rustyhorde/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,rustyhorde/cargo,vojtechkral/cargo,tsurai/cargo,rustyhorde/cargo,eminence/cargo,tsurai/cargo,rust-lang/cargo,tshepang/cargo,vojtechkral/cargo,vojtechkral/cargo,eminence/cargo,tshepang/cargo,rustyhorde/cargo,nipunn1313/cargo,tsurai/cargo,nipunn1313/cargo,vojtechkral/cargo,nipunn1313/cargo,nipunn1313/cargo