Rename `extensions` to `methods` in libs

[brson]

methods is shorter, maybe more descriptive

[graydon]

Agreed

[nikomatsakis]

Actually I'm wondering if both names aren't wrong. One problem I'm finding in practice is that if all impls have the same name, it's hard to sort out what you want to use and what you want to export.

Here is the scenario I'm talking about:

import result::{result, methods}; // something I use

export foo, methods;

type foo = ...;
impl methods for foo { ... }

I am wondering if it wouldn't be better to name the impl after the type in question. Then you could (easily) export just the methods for your type and not others, and you can use a type and its methods by doing:

import some_type::some_type;

All that said, I suppose that #1893 addresses this too:

import result::{result, methods}; // something I use

pub type foo = ...;
pub impl methods for foo { ... }

That's another benefit of #1893 I hadn't considered.

So I don't know, maybe methods is best after all---anyhow, just tossing this out there.

[graydon]

Closing this since separately-named impls are going away. Reopen if you think I misunderstood.