Make ciborium-ll usable standalone for std builds

[mpalmer]

Without this change, attempts to use anything in ciborium-ll that refers to ciborium_io::Write is basically unusable, because ciborium-io's std feature is never enabled, meaning that the types that implement io::std::Write aren't available for use, which leaves things in a rather tricky state.

Signed-off-by: Matt Palmer matt@hezmatt.org

[rjzak]

Please re-work the commit message so it follows the conventional commits specs https://www.conventionalcommits.org/en/v1.0.0/.

Example: feat: ciborium-ll usable standalone for std builds