Make FromStr::Err not need to be a Send+Sync+'static

[chabapok]

As far as I understand, standard rust library does not require error::Error to be a Send+Sync+'static, and (for example) structopt does not. But clap require. Thus, the library turns out to be incompatible with most parsing libraries that return Box<dyn Error> as an error.

This commit proposes to relax this requirement. This makes the library more backward-compatible with structopt. This will allow errors to be thrown from libraries that return errors, that not Send or Sync, for example ssсanf:

impl FromStr for MyConfigElement {
    type Err = sscanf::Error; // <<-- sscanf library return Box<dyn Error>, not Box<dyn Error+Send+Sync>
    // so sscanf::Error don't works with clap, but it's works in old 'structopt' crate

    fn from_str(s: &str) -> Result<Self, Self::Err> {

        #[derive(sscanf::FromScanf)]
        #[sscanf(format = "{foo}")]
        struct ParseHelper {
            foo: u32,
        }

	let result = sscanf::sscanf!(s, "{ParseHelper}")?;
	Ok(MyConfigElement::new(result.foo))
    }
}

A test+example of using such errors has also been added.

I know that a year ago there was a discussion about how the return Ok() result could also be !send+!sync+!clone (#3833), and I tried to do that - but it turned out that it required a lot of work.

[epage]

Per the contributing guide, changes like this need to be discussed in an issue first.

Considering this is a breaking change, I'm going to go ahead and close this. Feel free to make an issue and if we agree on the direction, we can then talk about strategies for going forward (ie unstable feature flag)