Fix string aliases in deriveCompare

[gavv]

Thanks for sharing this tool.

I've encountered a bug: if a field is a type alias to string, generated compare function will have this:

    if c := strings.Compare(this.Foo, that.Foo); c != 0 {
        return c
    }

which doesn't compile:

cannot use this.Foo (variable of type StringAlias) as string value in argument to strings.Compare

This PR fixes it by changing the generated code to:

    if c := strings.Compare(string(this.Foo), string(that.Foo)); c != 0 {
        return c
    }

[awalterschulze]

Thank you so much for the fix and unit test <3 Sorry for the long wait