Linkify more things in rustdoc

[mdinger]

A page like vec has many things that could be useful to link to. These should be possible to link to by clicking on:

• The Trait Implementations header
• impl's on a page.
• Methods at the top

On index it's hit or miss:

• Examples
• Associated Types
• Required Methods
• Implementors

On clone

• Provided Methods

I don't have any big use cases but when I want to link to these for something like this bug I can't link to any specific line. I gotta say go to vec page, scroll 75% to bottom. Squint hard to see it.

[joshtriplett]

Just ran into this today; I'd really like to have links from the impls in the Implementors section to the source containing the impl.

[steveklabnik]

Triage: we always need more links....