`decode_as` argument ordering

[tarcieri]

We have several decode_as methods which currently take a reader: &mut impl Reader, algorithm: Algorithm in that order. Most of these are unreleased outside of prereleases (KeypairData::decode_as is the only one in the stable public API).

I'm wondering if this ordering should be reversed so the algorithm comes first, and it reads as decode_as(algorithm, ... rather than decode_as(reader, ...

cc @baloo

[baloo]

I'm fine with either options