Implement choice enums

[kovaxis]

This PR implements #138.

Basically, it allows using simple fieldless enums as arguments by deriving FromArgValue:

#[derive(FromArgValue)]
enum MyChoiceEnum {
    Black,
    Yellow,
}

#[derive(FromArgs)]
struct Args {
    #[argh(option)]
    color: MyChoiceEnum,
}

Additionally:

• I convert the PascalCase names of enum variants into snake_case, so that the code above could be run as ./some_bin --color black or ./some_bin --color yellow.

• #[argh(name = "arbitrary_name") attributes are allowed to specify any desired name for each variant.

• I implemented better error messages like Error parsing option '--color' with value 'purple': expected "black" or "yellow"

• I did not implement listing available options in the help message, because that would require some heavier refactoring and I am not very familiar with the codebase yet.

• I added some basic documentation to the lib.rs top-level doc-comment, to the simple example and to the FromArgValue trait, as well as tests.

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[sadmac7000]

Looks like one of our workflow tests in broken. You can fix it by changing .github/workflows/rust.yml

[kovaxis]

Should be done. Is there anything left to do? I noticed there is no changelog.