fix: stdlib U256 native function verification workaround

[xudon9]

• Add pragma opaque to arithmetic functions that invoke native functions;
• Specification for them added (TODO: but doesn't seem to be used by MVP);
• U256.md updated.

[codecov]

Codecov Report

Merging #3174 (69c7273) into master (6aec59d) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3174      +/-   ##
==========================================
+ Coverage   32.08%   32.13%   +0.06%     
==========================================
  Files         503      503              
  Lines       46298    46298              
  Branches    21236    21236              
==========================================
+ Hits        14848    14873      +25     
+ Misses      17419    17195     -224     
- Partials    14031    14230     +199     

Flag	Coverage Δ
unittests	32.13% <ø> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...m/types/src/account_config/events/config_change.rs	66.67% <0.00%> (-33.33%)	⬇️
consensus/src/dummy.rs	69.57% <0.00%> (-13.04%)	⬇️
vm/natives/src/vector.rs	16.67% <0.00%> (-8.33%)	⬇️
executor/src/block_executor.rs	21.28% <0.00%> (-6.38%)	⬇️
vm/types/src/on_chain_config/mod.rs	47.95% <0.00%> (-5.47%)	⬇️
state/state-tree/src/mock/mod.rs	43.48% <0.00%> (-4.34%)	⬇️
config/src/helper.rs	50.99% <0.00%> (-3.92%)	⬇️
commons/crypto/src/multi_ed25519/multi_shard.rs	42.86% <0.00%> (-3.17%)	⬇️
consensus/src/consensus.rs	58.74% <0.00%> (-3.17%)	⬇️
vm/types/src/token/stc.rs	62.86% <0.00%> (-2.85%)	⬇️
... and 68 more

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