.into_string() does not work well with other error types

[blyxxyz]

Some programs parse inside a function that returns an anyhow-style error type or a custom error type. This doesn't play well with OsString::into_string(): its error type (OsString) can convert to anyhow::Error but not to most other error types.

• rres uses .into_string().unwrap().
• minoru-fediverse-crawler uses .into_string().map_err(|ostr| anyhow!("{}", ostr.to_string_lossy()))?.
• Another workaround would be .into_string().map_err(lexopt::Error::from)?.
• Yet another is .parse()?. This performs a copy.

A ValueExt::string(self) -> Result<String, lexopt::Error> method would solve this. (There may be a better name.)

Error's From<OsString> impl should then be kept, except perhaps in a release that already breaks most code for other reasons. But it should be removed from the README.

[blyxxyz]

I've implemented ValueExt::string() in 8b1bfc2.

[blyxxyz]

ValueExt::string() has been released in v0.3.0.