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/main.rs

@@ -4,6 +4,7 @@
 #![warn(clippy::use_self)]
 
 mod cli;
+mod error;
 mod eval;
 mod files;