wasmi_cli: Preserve WASI exit status and other minor improvements

[Robbepop]

Closes #675.

cc @yamt

[paritytech-cicd-pr]

BENCHMARKS

	NATIVE			WASMTIME
BENCHMARK	MASTER	PR	DIFF	MASTER	PR	DIFF	WASMTIME OVERHEAD
execute/ bare_call_0	1.44ms	1.47ms	⚪ 1.64%	1.25ms	1.23ms	🟢 -1.51%	🟢 -16%
execute/ bare_call_0/typed	1.04ms	1.04ms	⚪ 0.29%	861.69µs	860.81µs	⚪ -0.09%	🟢 -17%
execute/ bare_call_1	1.49ms	1.52ms	🔴 2.23%	1.52ms	1.47ms	🟢 -3.37%	🟢 -3%
execute/ bare_call_16	2.35ms	2.36ms	⚪ 0.07%	4.44ms	4.40ms	🟢 -0.67%	🟡 87%
execute/ bare_call_16/typed	1.55ms	1.56ms	⚪ 0.54%	2.42ms	2.36ms	🟢 -2.49%	🟡 51%
execute/ bare_call_1/typed	1.16ms	1.19ms	🔴 2.73%	1.26ms	1.26ms	⚪ 0.00%	🟢 6%
execute/ bare_call_4	1.62ms	1.67ms	🔴 2.34%	2.06ms	2.01ms	🟢 -2.28%	🟢 21%
execute/ bare_call_4/typed	1.15ms	1.15ms	⚪ 0.25%	1.36ms	1.27ms	🟢 -6.91%	🟢 10%
execute/ br_table	1.19ms	1.18ms	⚪ -1.62%	1.47ms	1.46ms	⚪ -0.61%	🟢 24%
execute/ count_until	641.81µs	642.81µs	⚪ 0.11%	2.29ms	2.29ms	⚪ 0.01%	🔴 256%
execute/ factorial_iterative	318.35µs	318.52µs	⚪ 0.16%	961.11µs	958.57µs	⚪ -0.22%	🔴 201%
execute/ factorial_recursive	652.49µs	648.67µs	⚪ -0.49%	1.32ms	1.30ms	🟢 -1.51%	🔴 101%
execute/ fib_iterative	1.40ms	1.41ms	⚪ -0.03%	5.16ms	5.16ms	⚪ 0.02%	🔴 266%
execute/ fib_recursive	5.91ms	6.30ms	🔴 6.55%	11.30ms	11.57ms	🔴 2.44%	🟡 84%
execute/ global_bump	1.04ms	1.04ms	⚪ -0.06%	2.62ms	2.66ms	🔴 1.55%	🔴 156%
execute/ global_const	684.66µs	684.64µs	⚪ -0.02%	2.42ms	2.68ms	🔴 9.28%	🔴 291%
execute/ host_calls	27.74µs	27.38µs	⚪ -1.29%	39.48µs	38.60µs	🟢 -2.28%	🟢 41%
execute/ memory_fill	1.20ms	1.20ms	⚪ 0.11%	4.18ms	4.20ms	⚪ 0.40%	🔴 250%
execute/ memory_sum	1.18ms	1.18ms	⚪ -0.89%	4.15ms	4.15ms	⚪ -0.09%	🔴 252%
execute/ memory_vec_add	2.40ms	2.41ms	⚪ 0.21%	7.76ms	7.76ms	⚪ -0.03%	🔴 223%
execute/ recursive_is_even	1.17ms	1.15ms	⚪ -1.44%	2.50ms	2.31ms	🟢 -7.43%	🔴 101%
execute/ recursive_ok	152.42µs	152.79µs	⚪ -0.20%	302.69µs	310.93µs	🔴 2.78%	🔴 104%
execute/ recursive_scan	182.57µs	184.59µs	🔴 1.16%	370.82µs	379.10µs	🔴 2.50%	🔴 105%
execute/ recursive_trap	15.03µs	15.90µs	🔴 6.18%	30.87µs	31.75µs	⚪ 2.37%	🟡 100%
execute/ regex_redux	533.39µs	562.88µs	🔴 5.50%	1.39ms	1.38ms	⚪ -0.14%	🔴 146%
execute/ rev_complement	460.56µs	462.62µs	⚪ 0.25%	1.42ms	1.42ms	⚪ -0.29%	🔴 207%
execute/ tiny_keccak	326.96µs	328.80µs	⚪ 0.74%	1.14ms	1.14ms	⚪ 0.04%	🔴 248%
execute/ trunc_f2i	692.89µs	692.34µs	⚪ 0.02%	2.14ms	2.14ms	⚪ -0.04%	🔴 209%
instantiate/ wasm_kernel	64.81µs	64.36µs	⚪ 1.59%	69.79µs	70.33µs	⚪ -0.19%	🟢 9%
translate/ erc1155	236.83µs	236.02µs	⚪ -0.42%	459.11µs	459.92µs	⚪ 0.23%	🟡 95%
translate/ erc20	114.63µs	114.87µs	⚪ 0.41%	221.60µs	221.09µs	⚪ -0.18%	🟡 92%
translate/ erc721	162.62µs	161.73µs	⚪ -0.66%	322.42µs	322.31µs	⚪ 0.00%	🟡 99%
translate/ spidermonkey	0.00ns	0.00ns	⚪ 0.00%	0.00ns	0.00ns	⚪ 0.85%	🟢 0%
translate/ wasm_kernel	4.06ms	4.07ms	⚪ 0.36%	8.31ms	8.33ms	⚪ 0.11%	🔴 105%

Link to pipeline