Fix `StarkProof::new_dummy()` (#234) #874

Job	Run time
rustfmt	14s
no-std (stable, wasm32-unknown-unknown)	27s
Check with --all-features	24s
no-std (nightly, wasm32-unknown-unknown)	36s
Check with --no-default-features	23s
Clippy	19s
Test Rust stable on ubuntu	1m 7s
Test Rust nightly on ubuntu	1m 13s
	4m 43s

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