Revert "ssh-key: temporarily remove `ed25519` feature (#249)" (#251) #584

Job	Run time
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	49s
test (1.73.0)	2m 52s
build (x86_64-apple-darwin, macos-latest, stable)	41s
no_std (1.73.0, thumbv7em-none-eabi)	58s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 28s
test (stable)	2m 27s
no_std (1.73.0, wasm32-unknown-unknown)	59s
no_std (stable, thumbv7em-none-eabi)	53s
no_std (stable, wasm32-unknown-unknown)	50s
	11m 57s

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