Introduce async `Prover` #1143

Job	Run time
Light checks	37s
Build for no-std (stable)	40s
Check prover	1m 5s
Check all features and all targets against the MSRV, except for winter-prover	1m 42s
Build for no-std (nightly)	41s
Test Rust stable on	1m 18s
Test Rust nightly on	1m 21s
	7m 24s

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