Add Itoa/Atoi overloads, add string-related methods to Std contract

[fyrchik]

Close #1928 .

[codecov]

Codecov Report

Merging #1943 (40d1dd0) into master (a4b54b2) will increase coverage by 0.07%.
The diff coverage is 83.07%.

@@            Coverage Diff             @@
##           master    #1943      +/-   ##
==========================================
+ Coverage   82.68%   82.76%   +0.07%     
==========================================
  Files         290      290              
  Lines       23377    23489     +112     
==========================================
+ Hits        19330    19440     +110     
+ Misses       2813     2811       -2     
- Partials     1234     1238       +4     

Impacted Files	Coverage Δ
pkg/interop/native/std/std.go	0.00% <0.00%> (ø)
pkg/core/native/std.go	96.65% <94.73%> (+0.84%)	⬆️
pkg/consensus/consensus.go	70.21% <0.00%> (-0.61%)	⬇️
pkg/services/oracle/request.go	52.41% <0.00%> (+5.51%)	⬆️
pkg/services/oracle/oracle.go	90.90% <0.00%> (+11.36%)	⬆️

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[roman-khimov]

I'm a little worried about the interop interface we provide, we don't have overloading, thus it starts to look strange.