`SmtProof`: add accessors #1002

Job	Run time
clippy nightly on ubuntu	51s
Verify the docs on stable	36s
build stable no-std for wasm32-unknown-unknown	15s
build nightly no-std for wasm32-unknown-unknown	24s
rustfmt nightly on ubuntu	14s
test stable on ubuntu with --features default,serde	47s
test stable on ubuntu with --no-default-features	45s
test nightly on ubuntu with --features default,serde	48s
test nightly on ubuntu with --no-default-features	46s
	5m 26s

Provide feedback