ssh-key: DRY out ECDSA impls with `macro_rules!` #400

Job	Run time
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	49s
build (x86_64-apple-darwin, macos-latest, stable)	1m 37s
no_std (1.65.0, thumbv7em-none-eabi)	12m 31s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 32s
no_std (1.65.0, wasm32-unknown-unknown)	12m 8s
test (1.65.0)	28m 56s
no_std (stable, thumbv7em-none-eabi)	7m 8s
test (stable)	20m 42s
no_std (stable, wasm32-unknown-unknown)	6m 59s
minimal-versions / minimal-versions	53s
	1h 33m 15s

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