Add implicit assertions for CodeArgument constraints

[leonard84]

This change is 

[codecov]

Codecov Report

Merging #956 into master will increase coverage by <.01%.
The diff coverage is 83.67%.

@@             Coverage Diff              @@
##             master     #956      +/-   ##
============================================
+ Coverage     75.99%   75.99%   +<.01%     
- Complexity     3534     3544      +10     
============================================
  Files           377      377              
  Lines         10746    10788      +42     
  Branches       1367     1374       +7     
============================================
+ Hits           8166     8198      +32     
- Misses         2103     2109       +6     
- Partials        477      481       +4

Impacted Files	Coverage Δ	Complexity Δ
...mework/mock/constraint/CodeArgumentConstraint.java	65% <54.54%> (-35%)	3 <2> (ø)
...g/spockframework/compiler/InteractionRewriter.java	70.71% <60%> (-0.47%)	50 <6> (+1)
...org/spockframework/compiler/DeepBlockRewriter.java	77.77% <96.96%> (+4.09%)	68 <14> (+9)	⬆️

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[marcphilipp]

While I'm not familiar enough with the existing implementation to give it a thorough review, I really like the direction this is going as a user. 👍

[marcphilipp]

I really like the added documentation. 👍

However, I think a few more tests for the new behavior would be good or am I missing sth.?