Why does `Encoder` take `W` by reference, while `Decoder` takes it by value?

[DCNick3]

I find it weird that this API asymmetry exists. Is there a reason behind this I can't see?

Btw, I think taking W by value will allow enforcing flushing on finish (kind of tangential to #77). Not sure if it will unnecessarily limit some use-cases or not 🤔

[danieleades]

from memory the asymmetry is introduced by the presence of the DecodeIter, for which the encoder has no analogue.
There may be a way around this through careful propagation of lifetimes. I'm unsure at this stage whether the approach i've taken here was necessary or just the path of least resistance.

If not for the above i would probably have made them both pass-by-reference since making them pass-by-value when this isn't strictly required feels needlessly constraining (i don't have a specific use-case in mind that this constrains by the way)

I would welcome a PR if you feel like experimenting with either or both of the approaches