This commit makes the JSON emitter use `-Z terminal-width` in the
"rendered" field of the JSON output.
Signed-off-by: NDavid Wood <david@davidtw.co>

- Pass around document_private a lot more
- Add tests
  + Add tests for intra-doc links to private items
  + Add ignored tests for warnings in reference links

- lint: add `feature_gate` field, allow specifying it in `declare_lint!`

- generalize the lint feature gate check

- librustdoc: generalize whitelisting of feature-gated lints

Fixes clippy::{cone_on_copy, filter_next, redundant_closure, single_char_pattern, len_zero,redundant_field_names, useless_format, identity_conversion, map_clone, into_iter_on_ref, needless_return, option_as_ref_deref, unused_unit, unnecessary_mut_passed}

This lets us specify the default at the options declaration point,
instead of using `.unwrap(default)` or `None | Some(default)` at some
use point far away. It also makes the code more concise.

For now, this is all the crate contains, but more
attribute logic & types will be moved there over time.

I believe rustdoc should not be conflating private items (visibility
lower than `pub`) and hidden items (attribute `doc(hidden)`). This
matters now that Cargo is passing --document-private-items by default
for bin crates. In bin crates that rely on macros, intentionally hidden
implementation details of the macros can overwhelm the actual useful
internal API that one would want to document.

This PR restores the strip-hidden pass when documenting private items,
and introduces a separate unstable --document-hidden-items option to
skip the strip-hidden pass. The two options are orthogonal to one
another.