Move arena definitions into their own crate: wasmi_arena

[Robbepop]

Closes #491.

[paritytech-cicd-pr]

BENCHMARKS

	NATIVE			WASMTIME
BENCHMARK	MASTER	PR	DIFF	MASTER	PR	DIFF	WASMTIME OVERHEAD
execute/ count_until	2.18ms	2.14ms	⚪ -1.55%	9.32ms	9.42ms	⚪ 0.98%	🔴 341%
execute/ factorial_iterative	1.67µs	1.67µs	⚪ 0.57%	3.33µs	3.39µs	⚪ -0.50%	🔴 103%
execute/ factorial_recursive	2.12µs	2.16µs	🔴 1.63%	4.30µs	4.22µs	🟢 -1.73%	🟡 96%
execute/ fib_iterative	10.05ms	10.40ms	🔴 2.80%	15.82ms	15.85ms	⚪ 0.23%	🟡 52%
execute/ fib_recursive	19.09ms	19.25ms	⚪ 0.83%	38.71ms	38.73ms	⚪ -0.16%	🔴 101%
execute/ global_bump	3.65ms	3.62ms	⚪ -0.98%	13.22ms	13.88ms	🔴 5.07%	🔴 284%
execute/ host_calls	32.77µs	33.79µs	⚪ 2.16%	110.25µs	105.73µs	🟢 -4.10%	🔴 213%
execute/ memory_fill	8.62ms	8.60ms	⚪ -0.14%	14.69ms	14.77ms	⚪ 0.49%	🟡 72%
execute/ memory_sum	7.82ms	7.88ms	⚪ 1.60%	14.96ms	14.95ms	⚪ 0.03%	🟡 90%
execute/ memory_vec_add	15.13ms	15.13ms	⚪ 0.00%	30.50ms	30.48ms	⚪ -0.31%	🔴 101%
execute/ recursive_is_even	2.77ms	2.79ms	⚪ 0.98%	6.65ms	6.53ms	🟢 -1.70%	🔴 134%
execute/ recursive_ok	407.92µs	420.50µs	🔴 2.79%	914.68µs	913.76µs	⚪ -0.14%	🔴 117%
execute/ recursive_scan	477.16µs	497.78µs	🔴 4.45%	1.30ms	1.31ms	⚪ 0.63%	🔴 162%
execute/ recursive_trap	43.34µs	47.49µs	🔴 10.58%	95.19µs	96.21µs	⚪ 1.12%	🔴 103%
execute/ regex_redux	3.44ms	3.43ms	⚪ -0.22%	5.08ms	5.05ms	⚪ -0.70%	🟢 47%
execute/ rev_complement	3.07ms	3.12ms	🔴 1.84%	4.96ms	4.98ms	⚪ 0.39%	🟡 60%
execute/ tiny_keccak	2.64ms	2.63ms	⚪ -0.18%	4.28ms	4.27ms	⚪ 0.10%	🟡 62%
execute/ trunc_f2i	3.60ms	3.69ms	🔴 2.58%	8.98ms	8.98ms	⚪ 0.06%	🔴 143%
instantiate/ wasm_kernel	76.98µs	76.60µs	⚪ -0.25%	138.31µs	104.05µs	🟢 -24.66%	🟢 36%
translate/ wasm_kernel	6.07ms	6.06ms	⚪ -0.32%	11.91ms	11.89ms	⚪ -0.11%	🟡 96%

Link to pipeline