* 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
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