Get rid of the distinction between `method` and `function` in rustdoc JSON output

[jyn514]

Rust has no concept of methods. Distinguishing between these makes no sense and is an artifact of how rustdoc represents these internally - consider for example the ambiguous case

pub struct Foo {}

impl Foo {
    pub fn bar() {}
}

We should remove this distinction before the format is stabilized.

cc @obi1kenobi @CraftSpider @camelid

[obi1kenobi]

The one thing I'd be careful about is consistency of terminology and intuition between the Rust book and rustdoc. The Rust book mentions methods as a kind of associated function, even though this is merely a human-made distinction and is not observed at layers below "human reading the code."

I'm completely in favor of not distinguishing between them in rustdoc and having the function kind cover both. I'd just suggest that we make sure the rustdoc JSON docs prominently state that in the JSON output, there's no difference between methods, non-method associated functions, and free functions.

[jyn514]

there's no difference between methods, non-method associated functions, and free functions.

I think we distinguish associated items by adding a parent field or something like that?

But yeah documenting that the kind is the same for all 3 seems fine.