Use base64 in JSON

[fyrchik]

There are some changes in JSON representation of transaction.Witness, smartcontract.Parameter and transaction.Script in NEO3.
This PR replaces hex-encoding with base64.
https://github.com/neo-project/neo/blob/master/src/neo/Network/P2P/Payloads/Witness.cs#L47
https://github.com/neo-project/neo/blob/master/src/neo/SmartContract/ContractParameter.cs#L150

There are also changes in NEP6 token format, I won't change it now, because it is strange.

[codecov]

Codecov Report

Merging #1015 into master will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1015      +/-   ##
==========================================
- Coverage   65.57%   65.52%   -0.05%     
==========================================
  Files         188      188              
  Lines       16002    15952      -50     
==========================================
- Hits        10493    10453      -40     
+ Misses       4994     4989       -5     
+ Partials      515      510       -5     

Impacted Files	Coverage Δ
pkg/core/transaction/witness.go	100.00% <ø> (+18.18%)	⬆️
pkg/rpc/response/result/contract_state.go	100.00% <ø> (+7.01%)	⬆️
pkg/core/transaction/transaction.go	78.10% <100.00%> (+1.12%)	⬆️
pkg/smartcontract/parameter.go	82.03% <100.00%> (+0.37%)	⬆️

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[roman-khimov]

These are all good, but a rebase is needed now.