Add utilities for reverse complement sequences

[RagnarGrootKoerkamp]

Probably both as an explicit PackedSeq -> PackedSeqVec, and in-place PackedSeq::rc(&mut self)?

And maybe also an iter_bp_rc? Although I haven't actually needed this yet.

And maybe also a separate fn rc_kmer(kmer: u64, k: usize) -> u64? although we don't really expose kmers as bits so far.
(Well, to_word does that, but to me it's more of a 'give a hashable identity' rather than 'give me a bit representation'.)