[FVM] Refactor event emission code

[janezpodhostnik]

Just unified the event emission code a bit.

There is one point that requires special attention. The event size reported when you emit a cadence event is the payload size, however using the EmitFlowEvent method used the event type + payload size for the size.

I think the correct behaviour is the original one (just using the payload size) as the payload should contain the type as well.

[sideninja]

Nice

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (ab5b93f) 56.30% compared to head (005ac82) 56.21%.

Files	Patch %	Lines
fvm/environment/event_emitter.go	52.38%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4982      +/-   ##
==========================================
- Coverage   56.30%   56.21%   -0.10%     
==========================================
  Files         963      962       -1     
  Lines       90184    89783     -401     
==========================================
- Hits        50776    50468     -308     
+ Misses      35610    35549      -61     
+ Partials     3798     3766      -32     

Flag	Coverage Δ
unittests	56.21% <54.54%> (-0.10%)	⬇️

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