Add Func::new constructor

[Robbepop]

Closes #658.

cc @tomaka

[paritytech-cicd-pr]

BENCHMARKS

	NATIVE			WASMTIME
BENCHMARK	MASTER	PR	DIFF	MASTER	PR	DIFF	WASMTIME OVERHEAD
execute/ bare_call_0	1.47ms	1.46ms	⚪ -0.83%	1.17ms	1.17ms	⚪ 0.49%	🟢 -20%
execute/ bare_call_0/typed	1.06ms	1.05ms	⚪ -1.12%	765.26µs	768.24µs	⚪ 0.42%	🟢 -27%
execute/ bare_call_1	1.53ms	1.50ms	⚪ -1.30%	1.37ms	1.37ms	⚪ 0.20%	🟢 -9%
execute/ bare_call_16	2.35ms	2.34ms	⚪ -0.62%	4.28ms	4.26ms	⚪ -0.42%	🟡 82%
execute/ bare_call_16/typed	1.59ms	1.60ms	⚪ 0.87%	2.21ms	2.23ms	⚪ 0.76%	🟢 40%
execute/ bare_call_1/typed	1.17ms	1.16ms	⚪ -0.99%	1.21ms	1.18ms	⚪ -2.19%	🟢 1%
execute/ bare_call_4	1.67ms	1.64ms	🟢 -2.24%	1.85ms	1.85ms	⚪ 0.30%	🟢 13%
execute/ bare_call_4/typed	1.15ms	1.15ms	⚪ 0.00%	1.23ms	1.23ms	⚪ -0.19%	🟢 6%
execute/ br_table	1.18ms	1.18ms	⚪ -0.10%	1.40ms	1.43ms	🔴 2.18%	🟢 21%
execute/ count_until	682.77µs	651.47µs	🟢 -4.60%	2.13ms	2.13ms	⚪ 0.06%	🔴 227%
execute/ factorial_iterative	334.71µs	330.00µs	🟢 -1.45%	900.86µs	906.96µs	⚪ 0.71%	🔴 175%
execute/ factorial_recursive	665.95µs	644.30µs	🟢 -3.39%	1.35ms	1.35ms	⚪ 0.40%	🔴 110%
execute/ fib_iterative	1.35ms	1.52ms	🔴 11.94%	5.01ms	5.00ms	⚪ -0.03%	🔴 230%
execute/ fib_recursive	6.06ms	5.94ms	🟢 -2.08%	11.89ms	11.89ms	⚪ -0.03%	🟡 100%
execute/ global_bump	979.80µs	963.94µs	🟢 -1.65%	2.59ms	2.56ms	🟢 -1.28%	🔴 166%
execute/ global_const	736.76µs	743.46µs	⚪ 0.86%	2.43ms	2.40ms	🟢 -1.48%	🔴 223%
execute/ host_calls	27.43µs	27.73µs	⚪ 1.25%	42.25µs	40.91µs	🟢 -4.10%	🟢 48%
execute/ memory_fill	1.26ms	1.31ms	🔴 4.10%	4.03ms	4.03ms	⚪ -0.17%	🔴 208%
execute/ memory_sum	1.22ms	1.30ms	🔴 6.89%	4.01ms	4.01ms	⚪ 0.14%	🔴 208%
execute/ memory_vec_add	2.49ms	2.51ms	⚪ 0.73%	7.63ms	7.64ms	⚪ 0.08%	🔴 204%
execute/ recursive_is_even	1.14ms	1.14ms	⚪ 0.12%	2.36ms	2.22ms	🟢 -5.80%	🟡 95%
execute/ recursive_ok	150.87µs	150.56µs	⚪ -0.17%	345.89µs	313.08µs	🟢 -9.48%	🔴 108%
execute/ recursive_scan	188.29µs	179.55µs	🟢 -4.75%	427.93µs	394.38µs	🟢 -7.70%	🔴 120%
execute/ recursive_trap	15.26µs	16.07µs	🔴 4.60%	34.98µs	30.80µs	🟢 -11.86%	🟡 92%
execute/ regex_redux	536.44µs	537.13µs	⚪ 0.09%	1.39ms	1.39ms	⚪ -0.60%	🔴 158%
execute/ rev_complement	480.23µs	489.75µs	🔴 2.11%	1.40ms	1.39ms	⚪ -0.32%	🔴 185%
execute/ tiny_keccak	331.31µs	351.70µs	🔴 6.22%	1.14ms	1.15ms	⚪ 0.15%	🔴 226%
execute/ trunc_f2i	722.31µs	725.31µs	⚪ 0.44%	2.29ms	2.15ms	🟢 -6.09%	🔴 196%
instantiate/ wasm_kernel	64.67µs	65.03µs	⚪ -0.65%	68.02µs	72.08µs	🔴 4.35%	🟢 11%
translate/ erc1155	210.66µs	208.10µs	🟢 -1.55%	385.68µs	386.38µs	⚪ 0.06%	🟡 86%
translate/ erc20	103.89µs	103.08µs	⚪ -0.91%	189.96µs	190.23µs	⚪ 0.01%	🟡 85%
translate/ erc721	148.37µs	145.80µs	🟢 -2.00%	276.41µs	276.86µs	⚪ 0.21%	🟡 90%
translate/ spidermonkey	0.00ns	72.23ms	⚪ -0.49%	0.00ns	0.00ns	⚪ 1.11%	🟢 -100%
translate/ wasm_kernel	3.80ms	3.77ms	⚪ -0.84%	7.12ms	7.17ms	⚪ 0.62%	🟡 90%

Link to pipeline