Document and refactor vm package

[st0012]

This PR is for addressing the 2 feedback I got from #841

1. the vm.New vm.New and v.ExecInstructions methods would profit from some minimal documentation, especially what all the "paths" actually mean. some of them seemed semi obvious..
2. same for RegisterExternalClass ExternalClass and vm.Method albeit those are relatively clear

[st0012]

@yannick can you give this a look?

[codecov]

Codecov Report

Merging #845 into master will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
+ Coverage   81.10%   81.11%   +0.01%     
==========================================
  Files          54       54              
  Lines        7420     7419       -1     
==========================================
  Hits         6018     6018              
+ Misses       1179     1178       -1     
  Partials      223      223              

Impacted Files	Coverage Δ
vm/class.go	86.07% <0.00%> (ø)
vm/vm.go	83.50% <58.82%> (+0.42%)	⬆️
native/db/db.go	61.00% <100.00%> (ø)
native/plugin/plugin.go	11.96% <100.00%> (ø)
native/ripper/ripper.go	73.18% <100.00%> (ø)

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