... also it deleted some redundant ones, and it got some things wrong,
and it was gonna delete some of the cross-crate docs.rs links we can't
do as intra-doc links so I passed `--no-favored`.

Note that this adds some more broken intra doc links when building
without std support, but that was already a thing and I expect people
who build their own docs without std support can handle it.

This time I gave up on sed and used ruby.

    find -name '*.rs' -exec ruby -i -p blah.rb {} +

with

    $_.gsub!(%r{
        https://doc\.rust-lang\.org/
        (?: stable/)?
        ((?:core | std | alloc)(?:/\S+?)*)
        /(\w+)\.(\w+)\.html}x
    ) {
      path, kind, name = $~.captures
      suffix = case kind
        when 'method' then '()'
        when 'macro' then '!'
        else ''
      end
      r = [path.gsub('/', '::'), '::', name, suffix].join
      STDERR.puts [path, kind, name, suffix, r].inspect
      r
    }
    $_.gsub!(%r{
        https://doc\.rust-lang\.org/
        (?: stable/)?
        ((?: core | std | alloc)(?:/\S+?)*)
        /(?:
          index\.html
          | $
          | (?= \#)
         )}x
    ) {
      path, _ = $~.captures
      r = path.gsub('/', '::')
      STDERR.puts [path, r].inspect
      r
    }

cargo intraconv doesn't seem to get them reliably and also plenty of
links to other crates aren't actually intra-doc because they're in
crates that don't depend (or only dev-depend, or only conditionally
depend) on those crates, so this wasn't very automated.

I tried to only convert docs.rs links to unconditional dependencies to
intra-crate links, but it's possible that some slipped through in either
direction.