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!

Cargo.lock

@@ -16,6 +16,11 @@ dependencies = [
  "winapi 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "anyhow"
+version = "1.0.31"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -58,6 +63,7 @@ name = "hexyl"
 version = "0.8.0"
 dependencies = [
  "ansi_term 0.12.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "anyhow 1.0.31 (registry+https://github.com/rust-lang/crates.io-index)",
  "atty 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "clap 2.33.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.71 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -118,6 +124,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"
 "checksum ansi_term 0.12.1 (registry+https://github.com/rust-lang/crates.io-index)" = "d52a9bb7ec0cf484c551830a7ce27bd20d67eac647e1befb56b0be4ee39a55d2"
+"checksum anyhow 1.0.31 (registry+https://github.com/rust-lang/crates.io-index)" = "85bb70cc08ec97ca5450e6eba421deeea5f172c0fc61f78b5357b2a8e8be195f"
 "checksum atty 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)" = "d9b39be18770d11421cdb1b9947a45dd3f37e93092cbf377614828a319d5fee8"
 "checksum bitflags 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "cf1de2fe8c75bc145a2f577add951f8134889b4795d47466a54a5c846d691693"
 "checksum clap 2.33.1 (registry+https://github.com/rust-lang/crates.io-index)" = "bdfa80d47f954d53a35a64987ca1422f495b8d6483c0fe9f7117b36c2a792129"