Fix NEF serialization

[fyrchik]

1. Introduce MethodToken.
2. Add reserved bytes.
3. Merge compiler and version fields.

Related #1644 .

[codecov]

Codecov Report

Merging #1672 (719dcef) into master (376c22a) will increase coverage by 0.00%.
The diff coverage is 94.28%.

@@           Coverage Diff           @@
##           master    #1672   +/-   ##
=======================================
  Coverage   82.56%   82.56%           
=======================================
  Files         237      238    +1     
  Lines       19469    19486   +17     
=======================================
+ Hits        16074    16089   +15     
- Misses       2391     2393    +2     
  Partials     1004     1004           

Impacted Files	Coverage Δ
pkg/smartcontract/nef/nef.go	86.56% <88.88%> (+0.62%)	⬆️
pkg/core/interop/context.go	94.44% <100.00%> (-0.11%)	⬇️
pkg/io/binaryReader.go	100.00% <100.00%> (ø)
pkg/smartcontract/nef/method_token.go	100.00% <100.00%> (ø)
pkg/consensus/consensus.go	68.13% <0.00%> (-0.64%)	⬇️

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