Make error formatting more generic

Now, some errors are generic over the FileId type, meaning I can now
pretty-print command line argument errors as well as normal errors with
the same machinery. This is accomplished via the Eprint trait, whose
generics are a tad complex but probably still fine.

src/error.rs

@@ -0,0 +1,28 @@
+use codespan_reporting::diagnostic::Diagnostic;
+use codespan_reporting::files::Files;
+use codespan_reporting::term::{self, Config};
+use termcolor::StandardStream;
+
+pub trait Eprint<'a, F: Files<'a>> {
+    fn eprint_diagnostic<'f: 'a>(
+        files: &'f F,
+        config: &Config,
+        diagnostic: &Diagnostic<F::FileId>,
+    ) {
+        let mut out = StandardStream::stderr(termcolor::ColorChoice::Auto);
+        if let Err(e) = term::emit(&mut out, config, files, diagnostic) {
+            panic!("Error while reporting error: {}", e);
+        }
+    }
+
+    fn eprint<'f: 'a>(&self, files: &'f F, config: &Config);
+}
+
+pub fn eprint_error<'a, 'f: 'a, F, E>(files: &'f F, e: &E)
+where
+    F: Files<'a>,
+    E: Eprint<'a, F>,
+{
+    let config = Config::default();
+    e.eprint(files, &config);
+}