tor-netdoc: Replace an IEFE with use of .and_then()

impl ItemArgumentParseable for DigestAlgoInSignature {

    fn from_args<'s>(args: &mut ArgumentStream<'s>) -> StdResult<Self, ArgumentError> {

        let v = if (|| {

            let s = args.clone().next()?;

        let v = if

            args.clone().next()

            // Treat it as a fingerprint if it doesn't have any non-hex characters

            // (including lowercase ones).  If we reuse this item for new algorithms

            // they should have at least one letter g-z in their name.

            .and_then(|s| {

            s.chars().all(|c| c.is_ascii_hexdigit()).then_some(())

        })()

            })

        .is_some()

        {

            // next argument looks enough like a fingerprint that we don't treat as an algo name