Auto merge of #125443 - nnethercote:rustfmt-use-decls, r=lcnr,cuviper…

…,GuillaumeGomez

rustfmt `use` declarations

This PR implements rust-lang/compiler-team#750, which changes how `use` declarations are formatted by adding these options to `rustfmt.toml`:
```
group_imports = "StdExternalCrate"
imports_granularity = "Module"
```

r? `@ghost`

src/lib.rs

@@ -279,12 +279,12 @@
 #![feature(rustdoc_internals)]
 // tidy-alphabetical-end
 
-use LabelText::*;
-
 use std::borrow::Cow;
 use std::io;
 use std::io::prelude::*;
 
+use LabelText::*;
+
 /// The text for a graphviz label on a node or edge.
 pub enum LabelText<'a> {
     /// This kind of label preserves the text directly as is.

src/tests.rs

@@ -1,9 +1,11 @@
-use super::LabelText::{self, EscStr, HtmlStr, LabelStr};
-use super::{render, Edges, GraphWalk, Id, Labeller, Nodes, Style};
 use std::io;
 use std::io::prelude::*;
+
 use NodeLabels::*;
 
+use super::LabelText::{self, EscStr, HtmlStr, LabelStr};
+use super::{render, Edges, GraphWalk, Id, Labeller, Nodes, Style};
+
 /// each node is an index in a vector in the graph.
 type Node = usize;
 struct Edge {