core: decouple native contracts from interop service

[AnnaShaleva]

Closes #1191.

neo-project/neo#1655 won't be ported in this PR.

[codecov]

Codecov Report

Merging #1229 into master will decrease coverage by 0.01%.
The diff coverage is 69.69%.

@@            Coverage Diff             @@
##           master    #1229      +/-   ##
==========================================
- Coverage   66.68%   66.66%   -0.02%     
==========================================
  Files         200      200              
  Lines       17184    17176       -8     
==========================================
- Hits        11459    11451       -8     
+ Misses       5112     5111       -1     
- Partials      613      614       +1     

Impacted Files	Coverage Δ
pkg/core/interops.go	100.00% <ø> (ø)
pkg/core/native/contract.go	83.87% <ø> (+2.51%)	⬆️
pkg/core/native/native_nep5.go	71.02% <0.00%> (ø)
pkg/rpc/client/nep5.go	6.95% <ø> (ø)
pkg/rpc/client/policy.go	58.97% <ø> (ø)
pkg/core/native/interop.go	63.15% <64.00%> (+6.01%)	⬆️
pkg/core/interop/context.go	100.00% <100.00%> (ø)
pkg/core/native/native_gas.go	69.69% <100.00%> (-0.46%)	⬇️
pkg/core/native/native_neo.go	35.57% <100.00%> (-0.22%)	⬇️
pkg/core/native/policy.go	70.93% <100.00%> (ø)
... and 1 more

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[fyrchik]

Some tests are also needed. There is already core/native_contract_test.go which can be extended.