vm, compiler: move Emit* functions to a separate package

[fyrchik]

Closes #449, #534 .

[codecov]

Codecov Report

Merging #642 into master will increase coverage by 0.57%.
The diff coverage is 85.83%.

@@            Coverage Diff            @@
##           master    #642      +/-   ##
=========================================
+ Coverage   64.32%   64.9%   +0.57%     
=========================================
  Files         126     125       -1     
  Lines       10597   10497     -100     
=========================================
- Hits         6817    6813       -4     
+ Misses       3477    3406      -71     
+ Partials      303     278      -25

Impacted Files	Coverage Δ
pkg/vm/emit/bigint.go	95.77% <ø> (ø)
pkg/core/blockchain.go	31.24% <100%> (+0.06%)	⬆️
pkg/vm/stack.go	91.45% <100%> (ø)	⬆️
pkg/smartcontract/contract.go	60% <100%> (+33.68%)	⬆️
pkg/vm/stack_item.go	75.64% <100%> (ø)	⬆️
pkg/vm/serialization.go	95.29% <100%> (ø)	⬆️
pkg/rpc/txBuilder.go	50% <64.51%> (+11.05%)	⬆️
pkg/vm/emit/emit.go	83.75% <83.75%> (ø)
pkg/compiler/codegen.go	86.78% <91.89%> (ø)	⬆️

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