Rustfmt: Support for use_nested_groups Feature

[cramertj]

Now that https://github.com/rust-lang/rust/issues/44494 has been stabilized, it would be nice to have a rustfmt option to reformat all imports into the use_nested_groups format.

[nikomatsakis]

We should decide what is the idiomatic way to use_nested_groups. For example, I had code like this:

use chalk_engine::{
    context,
    ExClause,
    hh::HhGoal,
};

use rustc::{
    infer::InferOk, 
    infer::canonical::{Canonical, CanonicalVarValues},
    traits::ProgramClause, 
    ty::subst::Kind,
    ty::{Binder, OutlivesPredicate, Region, Ty, TyCtxt},
};

which was reformatted into:

use chalk_engine::{context, ExClause, hh::HhGoal};

use rustc::{infer::InferOk, infer::canonical::{Canonical, CanonicalVarValues},
            traits::ProgramClause, ty::subst::Kind,
            ty::{Binder, OutlivesPredicate, Region, Ty, TyCtxt}};

The current output doesn't feel right to me; it's hard to skim the names etc, but I can see how rustfmt reached that point. I'm not sure if the style I was using is the "right" one, but I sort of like it: basically like having the first import (the crate) be special, and each line below is sort of like a use statement of its own.

[topecongiro]

cc https://github.com/rust-lang-nursery/fmt-rfcs/issues/24.

[cramertj]

@nrc Should this be reopened, or should we open a new issue to discuss the formatting that @nikomatsakis mentioned above?

[nrc]

@cramertj we should discuss on rust-lang-nursery/fmt-rfcs#24