Add an is_empty method when len is there

It is customary for Rust data container structures to have both
is_empty and len, methods, because is_empty can both more to the point
and also faster to implement.

Sometimes is_empty makes no sense, as it doesn't for
openpgp::types::Curve, which isn't a container structure. Arguably it
would be better to rename Curve::len to something like Curve::bits,
but as that would change the API, I've opted to tell clippy that not
having is_empty in this case is OK.

Found by clippy lint len_without_is_empty:
https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty

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