feat: use anyhow for more structured error messages and better error reporting flexibility

This significantly changes the structure of potential output of
top-level errors.  Before, errors that have a `source` were printed on a
single line, with the `source` omitted. Now, an error with a `source`
will be printed like:

```
Error: <`Display` impl of error>

Caused by:
    <`Display` impl of `source` error>
```

If there are multiple `source` errors in a chain, then it will be of the
form:

```
Error: <`Display` impl of error>

Caused by:
    0: <`Display` impl of first `source` error>
    1: <`Display` impl of `source` of `source` error>
    # etc.
```

Now, in practice this never happened before, since the only error
type that ever could return from `<binary>::run` was `std::io::Error`,
which has no `source`. However, `source`s can start being added if, say,
future work involved using `anyhow::Context::context` or custom errors
that implement `std::Error::source` are added to the codebase. I believe
this is a good choice going forward, but I definitely want to foster
discussion and acceptance for it in the open first!

src/bin/hexyl.rs

@@ -9,9 +9,11 @@ use clap::{App, AppSettings, Arg};
 
 use atty::Stream;
 
+use anyhow::{anyhow, Error as AnyhowError};
+
 use hexyl::{BorderStyle, Input, Printer};
 
-fn run() -> Result<(), Box<dyn std::error::Error>> {
+fn run() -> Result<(), AnyhowError> {
     let app = App::new(crate_name!())
         .setting(AppSettings::ColorAuto)
         .setting(AppSettings::ColoredHelp)
@@ -165,7 +167,7 @@ fn run() -> Result<(), Box<dyn std::error::Error>> {
 
     let mut printer = Printer::new(&mut stdout_lock, show_color, border_style, squeeze);
     printer.display_offset(display_offset);
-    printer.print_all(&mut reader)?;
+    printer.print_all(&mut reader).map_err(|e| anyhow!(e))?;
 
     Ok(())
 }
@@ -189,7 +191,7 @@ fn main() {
                 _ => (),
             }
         } else {
-            eprintln!("Error: {}", err);
+            eprintln!("Error: {:?}", err);
         }
         std::process::exit(1);
     }

src/lib.rs

@@ -408,7 +408,7 @@ impl<'a, Writer: Write> Printer<'a, Writer> {
     pub fn print_all<Reader: Read>(
         &mut self,
         mut reader: Reader,
-    ) -> Result<(), Box<dyn std::error::Error>> {
+    ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
         let mut buffer = [0; BUFFER_SIZE];
         'mainloop: loop {
             let size = reader.read(&mut buffer)?;