Mark Variable/Trigger constructors private

[miguelandres]

These constructors should not be used directly, this makes the use of Builders mandatory.

These constructors have far too many parameters, they are hard to use and error-prone, Builders make errors more obvious.

[codecov-io]

Current coverage is 66.15% (diff: 100%)

Merging #82 into develop will increase coverage by 0.37%

@@            develop        #82   diff @@
==========================================
  Files            44         44          
  Lines          1166       1164     -2   
  Methods           0          0          
  Messages          0          0          
  Branches        152        152          
==========================================
+ Hits            767        770     +3   
+ Misses          339        334     -5   
  Partials         60         60          

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