Fix `StarkProof::new_dummy()` #873

Job	Run time
rustfmt	13s
Clippy	20s
Check with --all-features	23s
no-std (stable, wasm32-unknown-unknown)	26s
Check with --no-default-features	27s
Test Rust stable on ubuntu	1m 9s
no-std (nightly, wasm32-unknown-unknown)	32s
Test Rust nightly on ubuntu	1m 14s
	4m 44s

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