Add examples to hex iterator constructors

[mpbagot]

The hex -> bytes iterators in the iter module are visible in the public api but don't have any code examples attached to them. While they're reasonably self-explanatory, some short example snippets on the constructors should make sure that it's absolutely unambiguous how they're to be used.

Add example to HexSliceToBytesIter::new and HexBytesToIter::from_pairs.

[apoelstra]

ACK 2e9594a; successfully ran local tests

[tcharding]

ACK 2e9594a

[tcharding]

The two lines of /// # } bothered me so I tried to refactor the test. Also to make it easier to read in the editor (I don't have syntax highlighting in my editor for rustdocs still). Totally optional, you have 2 acks already.

    /// ```
    /// # #[cfg(feature = "std")] {
    /// # use hex_conservative::HexToBytesIter;
    /// # fn main() -> Result<(), Box<dyn std::error::Error>> {
    ///
    /// let hex_digits: Vec<u8> = b"deadbeef".iter().copied().collect();
    /// let pairs = hex_digits.chunks_exact(2).map(|c| [c[0], c[1]]);
    ///
    /// let decoded:  = HexToBytesIter::from_pairs(pairs).collect::<Result<_, _>>()?;
    /// assert_eq!(decoded, [0xde, 0xad, 0xbe, 0xef]);
    ///
    /// # Ok(()) }}
    /// ```

[tcharding]

Oh my version renders poorly because of the new lines. Please ignore.