Allow splitting multi-line `use` statements into separate single-line statements

[Aaron1011]

By default, rustfmt currently formats the following code

use std::{ReallyLongImportTypeNameOne, ReallyLongImportTypeNameTwo, ReallyLongImportTypeNameThree, ReallyLongImportTypeNameFour};

as

use std::{
    ReallyLongImportTypeNameFour, ReallyLongImportTypeNameOne, ReallyLongImportTypeNameThree,
    ReallyLongImportTypeNameTwo,
};

it would be nice to have an option to split these imports into multiple lines, with each line respecting the maximum line width:

use std::{ReallyLongImportTypeNameFour, ReallyLongImportTypeNameThree};
use std::{ReallyLongImportTypeNameOne, ReallyLongImportTypeNameTwo};

[calebcartwright]

There's a ton of ongoing discussions about strategies/styles for imports, relevant issues linked below for reference. The gist though is that the default behavior isn't going to change, but there's going to be various config knobs for ordering, grouping, and granularity. I don't recall anyone asking for this particular style yet though should presumably be feasible for anyone sufficiently motivated

rust-lang/style-team#140
#3362, particularly #3362 (comment) and #3362 (comment)

(edit - fixed link)