Minor improvements for SIMDWrapper / zipping / event model

A few assorted fixes/improvements I came across while doing other things:

• LHCb::Pr::Fitted::Forward::Tracks: throw an exception instead of segfaulting if the ancestor relations are null
• SIMDWrapper: add a constexpr static std::size_t size() { ... } to sse::mask_v for consistency with other backends
• Zipping: change is_proxy_v<T> to not depend on an explicit is_proxy static variable
• Add std::any -> <any> to list of known headers

Should be tested with Rec!2141 (merged) and Moore!577 (merged)

cc: @apearce @mvesteri @nnolte