rustdoc: Footnotes don't work well when used on multiple doc comments that share page.

[aDotInTheVoid]

pub struct Foo;

impl Foo {
    /// Link 1 [^1]
    ///
    /// [^1]: Hiya
    pub fn l1(){}

    /// Link 2 [^2]
    ///
    /// [^2]: Biya
    pub fn l2() {}
}

Not only is this ugly, but both footnotes use the same link/number, despite being seperate in the source code.

I think the right thing to do here is to show all footnotes for a page in one place (at the bottom). This may have some subtle interactions with when markdown content appears on multiple pages (eg summaries, trait methods), but I've not looked into the details yet.

Origionly inspired by this output: