Implement function pointers in the VM

[fyrchik]

Closes #883.

[codecov]

Codecov Report

Merging #934 into master will decrease coverage by 0.08%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #934      +/-   ##
==========================================
- Coverage   64.05%   63.96%   -0.09%     
==========================================
  Files         196      196              
  Lines       16473    16561      +88     
==========================================
+ Hits        10551    10594      +43     
- Misses       5375     5415      +40     
- Partials      547      552       +5     

Impacted Files	Coverage Δ
pkg/vm/opcode/opcode_string.go	2.87% <0.00%> (-0.05%)	⬇️
pkg/compiler/codegen.go	85.65% <25.71%> (-2.42%)	⬇️
pkg/vm/stack_item.go	85.01% <54.05%> (-2.80%)	⬇️
pkg/vm/vm.go	82.33% <82.35%> (+<0.01%)	⬆️
pkg/compiler/func_scope.go	100.00% <100.00%> (ø)
pkg/vm/context.go	70.00% <100.00%> (ø)
pkg/vm/serialization.go	93.04% <0.00%> (+1.73%)	⬆️

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[roman-khimov]

We certainly need additional compiler tests and new issues to track what we're currently missing in terms of lamdas support.

[fyrchik]

@roman-khimov I have created an issue (#939), though there are no things which are really needed. This is more about support in VM.

[roman-khimov]

This one needs to be rebased (see the latest comment also, I think we're better referencing the script itself for now).