[WIP] TokenPartitionTree refactoring / "Choice" nodes support

[mglb]

Objectives

• Support multiple node types (subclasses) in TokenPartitionTree instead of one super-node
• Implement Choice node which directly maps to Choice operator in Layout Optimizer

Design decisions

TokenPartitionTree uses a variant for storing nodes instead of regular polymorphism

Main reason: I wanted to do something that would work without requiring bigger changes in code that uses TokenPartitionTree. Regular polymorphism would change node objects stored in Children list into pointers, which would require a lot of changes at once in the whole codebase.
With the use of a variant there was only one thing that required a change in the whole code base: Children() now returns pointer instead of reference, and can return nullptr. This is mostly change of . into ->. nullptr is handled mostly through the use of is_leaf() instead of Children()->empty().

Minor reason: this will allow linear storage of child nodes in memory. I didn't do any benchmarks, so I can only guess whether this helps much.

Custom Variant implementation

I've implemented Variant class from scratch instead of using std::variant.

Main reason: I wanted a possibility to "cast" pointer to object stored in variant into a pointer to the Variant holding it. This requires a guarantee that the variant's data member which stores the value is the first member, or at least access to this member in order to use offsetof. Neither is provided by std::variant.

This possibility is used in TokenPartition node implementations (which are stored in a Variant) to "cast" this into pointer to Variant, which is then assigned as parent pointer in child nodes.

[codecov-commenter]

Codecov Report

Merging #1328 (818c6b4) into master (b534c1f) will decrease coverage by 0.18%.
The diff coverage is 87.26%.

@@            Coverage Diff             @@
##           master    #1328      +/-   ##
==========================================
- Coverage   92.87%   92.69%   -0.19%     
==========================================
  Files         340      341       +1     
  Lines       23693    24048     +355     
==========================================
+ Hits        22006    22292     +286     
- Misses       1687     1756      +69     

Impacted Files	Coverage Δ
common/formatting/tree_unwrapper.h	91.66% <ø> (ø)
common/util/container_proxy.h	100.00% <ø> (ø)
common/util/logging.h	100.00% <ø> (ø)
common/util/value_saver.h	100.00% <ø> (ø)
common/util/vector_tree.h	98.66% <ø> (ø)
verilog/formatting/formatter.cc	93.62% <ø> (ø)
common/formatting/token_partition_tree.cc	87.69% <60.58%> (-10.64%)	⬇️
common/formatting/tree_unwrapper.cc	76.64% <62.50%> (+3.72%)	⬆️
common/formatting/align.cc	89.31% <85.71%> (ø)
common/formatting/token_partition_tree.h	86.30% <86.30%> (-13.70%)	⬇️
... and 12 more

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