Add link-menus for methods with multiple implementations

[jorendorff]

Currently [[Call]] is not a link, because there is not a single obvious place for that to link to.

But [[Call]] should be clickable. On consideration I think clicking it should bring up a menu of links:

[[Call]] method
…described (Table 9)
…of an ordinary Function object (8.3.16.1)
…of an exotic bound Function object (8.4.1.1)
…of an exotic Proxy object (8.5.14)

Like an index.

Making a real index is a lot of work; faking it is what fixup_links does.