The method which computes the weight for a `BasicBlockNode` now relies
on a new `WeightTraits` which is specialized for `BasicBlock` and for
`DotNode`.
In this way, we can change the implementation of the `getWeight` method
without having to modify the implementation inside the `revng` project.
`BasicBlockNode` and `RegionCFG` classes are now template classes. This
means that the `BasicBlockNode` class can be used as a generic wrapper
for any type of object in the original graph (it is usually used to wrap
a `llvm::BasicBlock *` for decompilation purposes, but in tests it can
be used to wrap a `DotNode` object) that implementes `GraphTraits`.
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