Update to derive-deftly 0.12.1
This adapts arti.git for the two incompatible changes Diziet/rust-derive-deftly#80 (closed) and Diziet/rust-derive-deftly#95 (closed)
I'm hoping these are the last changes needed in arti.git before derive-deftly 1.0, although there'll be at least two more semver bumps there.