Simplify imports by importing fewer items directly (#2404)

When reading the code without an IDE, I find it useful to use explicit
module names, especially when items come from the standard library.

So `io::Error` instead of just `Error`, especially when people have
just been told about `std::error::Error` as well.

I also omitted most single-use items: I find it has less cognitive
overhead to say “we import `fmt`” and then later use `fmt::Display`
and `fmt::Formatter` in the code. It’s clear from the name that these
two things have something to do with formatting.

Finally, I made a few usages more consistent so that we refer to each
item in the same way within a single codeblock.

src/error-handling/thiserror.md

@@ -9,8 +9,8 @@ avoid boilerplate when defining error types. It provides derive macros that
 assist in implementing `From<T>`, `Display`, and the `Error` trait.
 
 ```rust,editable,compile_fail
-use std::fs;
-use std::io::{self, Read};
+use std::io::Read;
+use std::{fs, io};
 use thiserror::Error;
 
 #[derive(Debug, Error)]

src/error-handling/try-conversions.md

@@ -28,9 +28,8 @@ higher-level errors.
 
 ```rust,editable
 use std::error::Error;
-use std::fmt::{self, Display, Formatter};
-use std::fs::File;
-use std::io::{self, Read};
+use std::io::Read;
+use std::{fmt, fs, io};
 
 #[derive(Debug)]
 enum ReadUsernameError {
@@ -40,8 +39,8 @@ enum ReadUsernameError {
 
 impl Error for ReadUsernameError {}
 
-impl Display for ReadUsernameError {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+impl fmt::Display for ReadUsernameError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self {
             Self::IoError(e) => write!(f, "I/O error: {e}"),
             Self::EmptyUsername(path) => write!(f, "Found no username in {path}"),
@@ -57,7 +56,7 @@ impl From<io::Error> for ReadUsernameError {
 
 fn read_username(path: &str) -> Result<String, ReadUsernameError> {
     let mut username = String::with_capacity(100);
-    File::open(path)?.read_to_string(&mut username)?;
+    fs::File::open(path)?.read_to_string(&mut username)?;
     if username.is_empty() {
         return Err(ReadUsernameError::EmptyUsername(String::from(path)));
     }