…ctor

The following code had issues because `autoload` was not calling into
the appropriate ractor autoload code:

```ruby
require "tmpdir"
rs = []
tmpdir = Dir.mktmpdir("autoload")
path = File.join(tmpdir, "a.rb")
File.open(path, 'w') do |f|
  f.write("class A; end\n")
end

100.times do
  rs << Ractor.new(path) do |path|
    autoload :A, path
  end
end

while rs.any?
  r, obj = Ractor.select(*rs)
  rs.delete(r)
end
```