spki

x509-cert: introduce a `builder::AsyncBuilder` trait #2696

Job	Run time
test (1.65.0)	2m 28s
build (1.65.0, thumbv7em-none-eabi)	1m 50s
test (stable)	1m 4s
build (1.65.0, wasm32-unknown-unknown)	1m 37s
build (stable, thumbv7em-none-eabi)	26s
build (stable, wasm32-unknown-unknown)	37s
minimal-versions / minimal-versions	1m 10s
	9m 12s

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