[fix] Introduce intermediate virtual nodes for select concrete nodes

[slarse]

Fix #132

This PR introduces virtual nodes to create a layer of indirection between certain complex nodes and their child lists. This is necessary where nodes have varying amounts of certain types of children, and these children are adjacent in the "flat" representation, as otherwise there may be conflicts across syntactical boundaries.

Only a select few nodes are "exploded" with virtual nodes. Currently, it is only subtypes of CtType, CtExecutable and CtExecutableReference. To add a new type, simply add it to the list in NodeFactory.EXPLODED_TYPES.

[codecov-commenter]

Codecov Report

Merging #133 into master will increase coverage by 0.26%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master     #133      +/-   ##
============================================
+ Coverage     80.11%   80.37%   +0.26%     
- Complexity      443      471      +28     
============================================
  Files            36       36              
  Lines          1624     1717      +93     
  Branches        243      260      +17     
============================================
+ Hits           1301     1380      +79     
- Misses          209      217       +8     
- Partials        114      120       +6     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/se/kth/spork/base3dm/Pcs.java	68.00% <0.00%> (-5.92%)	10.00 <0.00> (ø)
...java/se/kth/spork/spoon/matching/SpoonMapping.java	85.88% <50.00%> (-0.87%)	38.00 <0.00> (+1.00)	⬇️
...h/spork/spoon/pcsinterpreter/SporkTreeBuilder.java	79.84% <78.57%> (-0.67%)	34.00 <8.00> (+5.00)	⬇️
.../java/se/kth/spork/spoon/wrappers/NodeFactory.java	78.44% <84.72%> (+9.21%)	18.00 <12.00> (+13.00)
src/main/java/se/kth/spork/base3dm/ChangeSet.java	94.11% <100.00%> (ø)	29.00 <1.00> (ø)
src/main/java/se/kth/spork/base3dm/ListNode.java	100.00% <100.00%> (+33.33%)	6.00 <1.00> (+2.00)
src/main/java/se/kth/spork/spoon/PcsBuilder.java	100.00% <100.00%> (ø)	12.00 <8.00> (+5.00)
...kth/spork/spoon/matching/ClassRepresentatives.java	94.73% <100.00%> (+0.19%)	10.00 <0.00> (+2.00)
...h/spork/spoon/pcsinterpreter/SpoonTreeBuilder.java	87.50% <100.00%> (+0.09%)	41.00 <0.00> (ø)
... and 1 more

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[slarse]

The PR fails the file merge benchmark as, for some reason, the changes cause previously implicit type parameters to become explicit. This does not seem to be too much of a problem in practice, I don't think it affects anything other than textual accuracy.

[slarse]

I think the deteriorated score is related to virtual nodes that do not have children not getting the leaf child list as described in #116