Command line utility

For now it can only parse an end entity DER certificate. Can be
extended further later.

Is not meant to be public, does not provide a stable interface.

Useful mostly for debugging of WebPKI itself.

```
% cargo run -p webpki-bin -- print-cert ./tests/netflix/ca.der
...
target/debug/webpki: failed to parse a cert ./tests/netflix/ca.der: BadDER
zsh: exit 1     cargo run -p webpki-bin -- print-cert ./tests/netflix/ca.der

% cargo run -p webpki-bin -- print-cert ./tests/netflix/ee.der
...
cert ./tests/netflix/ee.der parsed successfully
```

I agree to license my contributions to each file under the terms
given at the top of each file I changed.

Cargo.toml

@@ -88,3 +88,9 @@ rpath = false
 lto = true
 debug-assertions = false
 codegen-units = 1
+
+[workspace]
+members = [
+    ".",
+    "webpki-bin",
+]

webpki-bin/Cargo.toml

@@ -0,0 +1,27 @@
+# Copyright 2021 Brian Smith.
+#
+# Permission to use, copy, modify, and/or distribute this software for any
+# purpose with or without fee is hereby granted, provided that the above
+# copyright notice and this permission notice appear in all copies.
+#
+# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
+# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR
+# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+[package]
+authors = ["Brian Smith <[email protected]>"]
+description = "Test binaries for webpki crate."
+edition = "2018"
+license-file = "../LICENSE"
+name = "webpki-bin"
+repository = "https://github.com/briansmith/webpki"
+version = "0.0.0"
+publish = false
+
+[dependencies]
+webpki = { version = "*", path = "..", features = ["std"] }
+clap = "3.0.0-beta.2"

webpki-bin/src/bin/webpki.rs

@@ -0,0 +1,61 @@
+use clap::Clap;
+use std::convert::TryFrom;
+use std::env;
+use std::fs;
+use std::path::PathBuf;
+use std::process;
+
+#[derive(Clap)]
+#[clap(about = "Parse and print a certificate")]
+struct PrintCert {
+    filename: PathBuf,
+}
+
+#[derive(Clap)]
+enum Command {
+    PrintCert(PrintCert),
+}
+
+#[derive(Clap)]
+#[clap(about = "Utility to debug webpki. Does not have a stable API.")]
+struct Opts {
+    #[clap(subcommand)]
+    command: Command,
+}
+
+fn print_error_and_exit(s: String) -> ! {
+    let arg0 = env::args().next();
+    let arg0 = arg0.as_ref().map(|s| s.as_str()).unwrap_or("webpki");
+    eprintln!("{}: {}", arg0, s);
+    process::exit(1)
+}
+
+fn print_cert(args: PrintCert) {
+    let der = match fs::read(&args.filename) {
+        Ok(der) => der,
+        Err(e) => print_error_and_exit(format!(
+            "cannot read file {}: {}",
+            args.filename.display(),
+            e
+        )),
+    };
+
+    match webpki::EndEntityCert::try_from(der.as_slice()) {
+        Ok(cert) => cert,
+        Err(e) => print_error_and_exit(format!(
+            "failed to parse a cert {}: {}",
+            &args.filename.display(),
+            e
+        )),
+    };
+
+    eprintln!("cert {} parsed successfully", &args.filename.display());
+    // TODO: actually print something about the certificate
+}
+
+fn main() {
+    let opts: Opts = Opts::parse();
+    match opts.command {
+        Command::PrintCert(args) => print_cert(args),
+    }
+}