Add package metadata flags to rand_core to document OsRng on docs.rs

[hdevalence]

The rand_core Cargo.toml doesn't specify std as a default feature (fine) but also doesn't specify it with docs.rs metadata fields, so when looking at the rand_core docs, the OsRng type does not appear anywhere in the rendered docs. This is confusing to match up with code that uses rand_core::OsRng.

Adding

[package.metadata.docs.rs]
features = ["std"]

is one potential fix to this issue.

[newpavlov]

I think we can close this issue as a duplicate of #986.

[dhardy]

More like a duplicate of #1001. @hdevalence's suggestion fixes this and is mostly orthogonal to #986. Never mind, I'll open a PR.

[dhardy]

Or rather, I already did.