Introduce `SmtProof` #991

Job	Run time
Verify the docs on stable	1m 0s
clippy nightly on ubuntu	52s
rustfmt nightly on ubuntu	15s
build stable no-std for wasm32-unknown-unknown	20s
build nightly no-std for wasm32-unknown-unknown	23s
test stable on ubuntu with --features default,serde	42s
test stable on ubuntu with --no-default-features	49s
test nightly on ubuntu with --features default,serde	54s
test nightly on ubuntu with --no-default-features	44s
	5m 59s

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