cmpv2

x509-cert: introduce a `builder::AsyncBuilder` trait #893

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 29s
build (1.65.0, wasm32-unknown-unknown)	1m 28s
build (stable, thumbv7em-none-eabi)	27s
build (stable, wasm32-unknown-unknown)	32s
test (1.65.0)	1m 30s
test (stable)	1m 48s
minimal-versions / minimal-versions	5m 44s
	12m 58s

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