Move v2::Particle types from Phys to LHCb, add new zip proxy types

Note this currently targets the branch of !2621 (merged); it should not be merged into there, we can change the target to master once !2621 (merged) is merged.

• Import @nnolte's v2::Composites and friends from Phys into LHCb.
• Add SOACollection helper for implementing dynamically-resizeable collections of columns ("a better struct-of-vectors")
• Use this helper to implement v2::Composites
• Modify the proxies produced by LHCb::Pr::Zip to support more different behaviours
  • In addition to the old "load" behaviour, where a SIMD proxy would yield contiguous elements, "broadcast" proxies (which yield a SIMD vector full of the same element) and "gather" proxies (which take a SIMD vector of indices) are now supported
• Add some extra helpers for implementing the proxy classes

cc: @nnolte

Phys!729 (merged) should be applied simultaneously.