Implement `StarkProof::new_dummy()` #860

Job	Run time
no-std (stable, wasm32-unknown-unknown)	1m 1s
rustfmt	12s
Clippy	20s
no-std (nightly, wasm32-unknown-unknown)	34s
Check with --all-features	26s
Check with --no-default-features	31s
Test Rust stable on ubuntu	1m 6s
Test Rust nightly on ubuntu	1m 16s
	5m 26s

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