Implement fmt::Display for enums, flags, and ints (#18)

* Implement fmt::Display for enums

`wasi_common` relies on `strerror` to nicely format error messages.
`strerror` is autoimplemented in `wig`. I thought it might be useful
to provide a Rust-idiomatic alternative which boils down to autoimplementing
`fmt::Display` for all enums.

* Implement fmt::Display for flags

* Implement fmt::Display for ints

tests/errno.witx

@@ -1,8 +1,13 @@
 (typename $errno
     (enum u32
+        ;;; Success
         $ok
+        ;;; Invalid argument
         $invalid_arg
+        ;;; I really don't want to
         $dont_want_to
+        ;;; I am physically unable to
         $physically_unable
+        ;;; Well, that's a picket line alright!
         $picket_line))