core: port StdLib's hexEncode, hexDecode and Policy's getBlockedAccounts APIs

[Turalchik]

Close #3990.
Close #3992.

[codecov]

Codecov Report

❌ Patch coverage is 88.57143% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.06%. Comparing base (08aaef3) to head (a5e223e).
⚠️ Report is 18 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/rpcclient/policy/policy.go	81.81%	4 Missing and 2 partials ⚠️
pkg/core/native/policy.go	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4004      +/-   ##
==========================================
+ Coverage   82.04%   82.06%   +0.01%     
==========================================
  Files         351      351              
  Lines       50035    50109      +74     
==========================================
+ Hits        41051    41120      +69     
- Misses       7255     7258       +3     
- Partials     1729     1731       +2     

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[AnnaShaleva]

Review is not finished yet, PR was updated.

[AnnaShaleva]

A common note: if some functionality/doc is already implemented, then follow the established style of the code/comments/docs. Don't make tests or code complicated, try to keep the code simple and readable. Try to avoid long variable names, try to keep the names meaningful only if these variables are important.

These rules make the codebase maintenance process easier, it's especially important for large codebases.

[AnnaShaleva]

Good.