Rolling up PRs in the queue

Closes #14675 (rustc: Encode argument names for traits)
Closes #14681 (rustc: Avoid UB with signed division/remainder)
Closes #14682 (librustc: Update AutoObject adjustment in writeback.)
Closes #14683 (Avoid 16-byte filenames in rlibs)
Closes #14687 (rustdoc: Inline static documentation across crates)
Closes #14689 (Remove reference to ~str in documentation)
Closes #14692 (Rename Iterator::len to count)
Closes #14693 (Implement Eq for HashSet and HashMap)
Closes #14699 (url: encode small bytes correctly.)
Closes #14700 (rustdoc: Submit examples to play.rust-lang.org)
Closes #14701 (mk: Run doc tests with --cfg dox)
Closes #14710 (rustc: Preserve reachable extern fns with LTO)
Closes #14711 (Removing unused wrapper to libc::close.)