Remove unnecessary lifetime from CRC digest in deserializer

[MathiasKoch]

This PR is the deserialize complement to https://github.com/jamesmunns/postcard/pull/129

This is required to allow wrappers such as

fn deserialize(buf: &[u8]) -> Result<Data<'_>, Error> {
    let crc = Crc::<u32>::new(&CRC_32_ISCSI);
    let digest = crc.digest();
    postcard::from_bytes_crc32(buf, digest).map_err(|_| Error::Deserialize)
}

without getting

   |
54 |         let digest = crc.digest();
   |                      --- `crc` is borrowed here
55 |         postcard::from_bytes_crc32(buf, digest).map_err(|_| Error::Deserialize)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returns a value referencing data owned by the current function

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[MathiasKoch]

Nevermind this.. Moved a bit too fast with the PR.. Don't think this will work in practice?