Give in-crate reason about why draft features are being used in a crate

We set the crate specification version to draft versions, if draft features are being used. If we would document which feature is being used, when exporting a crate we could check if, in the meanwhile, this feature is being stabilized and move to stable context versions. We could use an extra entity for machine-readable documentation.

Benefit would be automated conversion to stable versions, if available.

No files, tests, or entry points are named. Start by tracing where the crate specification version is assigned and where crates are exported; define how draft-feature usage is recorded in machine-readable metadata and how stabilization would enable a stable version.

Written by the indexing model from the issue text.