Add Pages::checked_sub method

[Robbepop]

Closes #659.

This mirrors the already existing Pages::checked_add method.

@tomaka

[paritytech-cicd-pr]

BENCHMARKS

	NATIVE			WASMTIME
BENCHMARK	MASTER	PR	DIFF	MASTER	PR	DIFF	WASMTIME OVERHEAD
execute/ bare_call_0	1.54ms	1.50ms	🟢 -2.67%	1.16ms	1.16ms	⚪ -0.42%	🟢 -23%
execute/ bare_call_0/typed	1.06ms	1.06ms	⚪ -0.65%	759.50µs	758.76µs	⚪ -0.10%	🟢 -28%
execute/ bare_call_1	1.59ms	1.60ms	🔴 1.18%	1.34ms	1.33ms	🔴 -1.09%	🟢 -17%
execute/ bare_call_16	2.45ms	2.42ms	⚪ -1.54%	4.18ms	4.18ms	⚪ 0.09%	🟡 72%
execute/ bare_call_16/typed	1.59ms	1.61ms	⚪ 0.82%	2.27ms	2.27ms	⚪ -0.29%	🟢 41%
execute/ bare_call_1/typed	1.17ms	1.23ms	🔴 4.47%	1.18ms	1.19ms	🔴 1.83%	🟢 -3%
execute/ bare_call_4	1.77ms	1.67ms	🟢 -5.60%	1.97ms	1.96ms	⚪ -0.23%	🟢 17%
execute/ bare_call_4/typed	1.18ms	1.18ms	⚪ 0.08%	1.23ms	1.25ms	⚪ 0.93%	🟢 6%
execute/ br_table	1.19ms	1.32ms	🔴 11.08%	1.43ms	1.44ms	⚪ 1.23%	🟢 9%
execute/ count_until	683.22µs	682.53µs	⚪ -0.09%	2.15ms	2.14ms	⚪ -0.58%	🔴 213%
execute/ factorial_iterative	328.24µs	333.10µs	🔴 1.37%	912.81µs	908.22µs	⚪ -0.51%	🔴 173%
execute/ factorial_recursive	648.92µs	647.92µs	⚪ -0.16%	1.42ms	1.34ms	🟢 -5.42%	🔴 108%
execute/ fib_iterative	1.35ms	1.36ms	⚪ 0.89%	5.03ms	5.02ms	⚪ -0.22%	🔴 268%
execute/ fib_recursive	5.88ms	6.01ms	🔴 2.38%	12.02ms	12.63ms	🔴 5.14%	🔴 110%
execute/ global_bump	959.83µs	960.11µs	⚪ -0.07%	2.57ms	2.56ms	⚪ -0.24%	🔴 167%
execute/ global_const	736.69µs	736.94µs	⚪ -0.05%	2.39ms	2.40ms	⚪ 0.26%	🔴 226%
execute/ host_calls	27.69µs	28.98µs	⚪ 3.26%	39.46µs	38.85µs	⚪ -1.60%	🟢 34%
execute/ memory_fill	1.26ms	1.25ms	⚪ -0.23%	4.03ms	4.03ms	⚪ -0.04%	🔴 221%
execute/ memory_sum	1.22ms	1.22ms	⚪ 0.04%	4.01ms	4.01ms	⚪ -0.02%	🔴 229%
execute/ memory_vec_add	2.50ms	2.60ms	🔴 3.88%	7.62ms	7.62ms	⚪ 0.00%	🔴 193%
execute/ recursive_is_even	1.15ms	1.13ms	⚪ -1.22%	2.19ms	2.19ms	⚪ 0.14%	🟡 94%
execute/ recursive_ok	153.23µs	152.56µs	⚪ -0.42%	310.07µs	311.22µs	⚪ 0.37%	🔴 104%
execute/ recursive_scan	183.18µs	182.46µs	⚪ -0.43%	390.59µs	390.71µs	⚪ -0.05%	🔴 114%
execute/ recursive_trap	15.09µs	15.12µs	⚪ 0.30%	30.99µs	31.93µs	🔴 3.00%	🔴 111%
execute/ regex_redux	537.25µs	534.28µs	⚪ -0.52%	1.39ms	1.37ms	🟢 -1.32%	🔴 156%
execute/ rev_complement	477.43µs	482.52µs	⚪ 1.05%	1.39ms	1.39ms	⚪ -0.14%	🔴 188%
execute/ tiny_keccak	347.15µs	332.28µs	🟢 -3.83%	1.14ms	1.15ms	⚪ 0.23%	🔴 245%
execute/ trunc_f2i	723.33µs	722.42µs	⚪ -0.04%	2.14ms	2.14ms	⚪ 0.00%	🔴 196%
instantiate/ wasm_kernel	64.51µs	64.38µs	⚪ -0.46%	71.33µs	72.05µs	🔴 1.90%	🟢 12%
translate/ erc1155	207.42µs	208.42µs	⚪ 0.16%	385.40µs	387.90µs	⚪ 0.64%	🟡 86%
translate/ erc20	102.87µs	102.90µs	⚪ -0.15%	188.77µs	191.11µs	🔴 1.33%	🟡 86%
translate/ erc721	147.13µs	145.85µs	⚪ -1.03%	275.84µs	277.01µs	⚪ 0.37%	🟡 90%
translate/ spidermonkey	0.00ns	0.00ns	⚪ -0.31%	0.00ns	0.00ns	🔴 1.82%	🟢 0%
translate/ wasm_kernel	3.75ms	3.75ms	⚪ -0.16%	7.10ms	7.17ms	⚪ 0.95%	🟡 91%

Link to pipeline

[Robbepop]

@tomaka If the API as implemented in this PR suffices you, I will merge.

[tomaka]

Yes, in addition to the impl From<Pages> for u32 that already exists this is good for me 👍