core: make GetUnspentCoins interop return array, fix #978.

[fyrchik]

Fix #978.

[codecov]

Codecov Report

Merging #979 into master-2.x will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@              Coverage Diff               @@
##           master-2.x     #979      +/-   ##
==============================================
- Coverage       68.00%   67.97%   -0.04%     
==============================================
  Files             145      145              
  Lines           14313    14320       +7     
==============================================
  Hits             9734     9734              
- Misses           4119     4126       +7     
  Partials          460      460              

Impacted Files	Coverage Δ
pkg/core/interop_neo.go	57.75% <0.00%> (-0.77%)	⬇️

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[roman-khimov]

1. Please fix the issue number (it's 978, not 938)
2. Our UnspentCoin structure differs from the one from C# (it kinda merges spent and unspent coins from it), you need to filter States by State to get the same result (unspents only).
3. We need to get back to a587274 then and re-add this function as GetUnspentCoins(t Transaction) []output.Output.