feat(VM): propagate values from immutable variables

[petar-dambovaliev]

If variables have constant values of a primitive type and they are never mutated (or mutated by constant expressions),
just like values from constants are currently propagated, we can do the same for this set of variables.
This is an optimization to avoid computation like searching for the value through the name valuepath during runtime.

Conditions for propagation

• There is no mutation
• There are no references given out
• The initial value during definition is a literal
• The type of the value is primitive

This must be done in 2 passes.
First pass needs to establish which variables adhere to these conditions and mark them.
The second pass can do this propagation based on that metadata.

Here is an example.

func Foo(b int) {
   a := 5
   
   if a > 1 {
      println(a)
   } 
}

This code would turn into

func Foo(b int) {
   a := 5
   
   if 5 > 1 {
      println(5)
   } 
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.84%. Comparing base (8a62a28) to head (c81683b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2801      +/-   ##
==========================================
- Coverage   60.95%   60.84%   -0.12%     
==========================================
  Files         564      563       -1     
  Lines       75273    75137     -136     
==========================================
- Hits        45884    45717     -167     
- Misses      26019    26055      +36     
+ Partials     3370     3365       -5     

Flag	Coverage Δ
contribs/gnofaucet	14.46% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (+0.35%)	⬆️
tm2	62.10% <ø> (+0.02%)	⬆️

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