Add InstructionPtr abstraction (take 2)

[Robbepop]

Closes #479.

[paritytech-cicd-pr]

CRITERION BENCHMARKS

BENCHMARK	MASTER	PR	Diff
execute/count_until	2.0961 ms	2.0421 ms	🟢 -2.4563 %
execute/factorial_iterative	1.7098 µs	1.523 µs	🟢 -10.8765 %
execute/factorial_recursive	2.2433 µs	2.0744 µs	🟢 -7.5794 %
execute/fib_iterative	10.19 ms	9.8127 ms	🟢 -3.4878 %
execute/fib_recursive	20.4881 ms	18.6807 ms	🟢 -9.7933 %
execute/global_bump	3.8591 ms	3.5449 ms	🟢 -7.9705 %
execute/host_calls	35.2978 µs	33.3171 µs	🟢 -5.5321 %
execute/memory_fill	9.4644 ms	8.1934 ms	🟢 -13.3865 %
execute/memory_sum	9.1463 ms	7.9282 ms	🟢 -13.3186 %
execute/memory_vec_add	16.0485 ms	14.8528 ms	🟢 -7.3883 %
execute/recursive_is_even	3.3526 ms	2.6447 ms	🟢 -20.9188 %
execute/recursive_ok	404.863 µs	390.0931 µs	🟢 -3.5266 %
execute/recursive_scan	523.6289 µs	485.4884 µs	🟢 -7.5352 %
execute/recursive_trap	39.4519 µs	39.285 µs	⚪ -0.332 %
execute/regex_redux	3.5605 ms	3.3738 ms	🟢 -5.4045 %
execute/rev_complement	3.1501 ms	3.0066 ms	🟢 -4.5125 %
execute/tiny_keccak	2.6625 ms	2.6299 ms	🟢 -1.2246 %
execute/trunc_f2i	3.7005 ms	3.5182 ms	🟢 -4.7955 %
instantiate/wasm_kernel	76.7078 µs	77.726 µs	⚪ +1.3078 %
translate/wasm_kernel	6.077 ms	6.1862 ms	⚪ +1.453 %

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