Add compile-time polymorphism for distributions for on-device separable user sampling

[sethrj]

Related to #2122 , add the infrastructure to sample from arbitrary distributions on device

• Add inp directory to corecel (Grid input should be moved there)
• Move distribution inputs from celeritas/inp/Events.hh to corecel/inp/Distributions.hh
• For each sampling type (1D, 3D, size_type/ParticleId, ...?), define a distribution enum and type dispatch for samplers similar to orange/surf but improved with our latest techniques
  • Define a DistributionCollectionGroup that has enums, offsets, records (needed for dynamic-sized distributions), and reals/ints (for backend storage)
  • Write setup-time DistributionInserter that takes inp::... and adds to the collection group, returning distribution ID for that type
  • Write compile-time DistributionVisitor that takes NativeCRef<DistributionCollectionGroup>, locally constructs the distribution, and returns a sample given an RNG
• Update primary generators to use on-device sampling