Need some adaptor for transforming plain variable / fancy pointer to raw pointer

[saki7]

I think we can make the raw pointer obtaining functor and range adaptor independent so that each component can be used in distinct places.

Naming candidates:

• Raw pointer obtaining functor:
  • iris::as_address(v) -- maybe confusing with std::addressof and std::to_address? They are already confusing, so we might not want to add the third candidate.
  • iris::as_raw_ptr(v) -- feels better, but somewhat verbose?
• Range adaptor:
  • Depends on the functor's name; iris::views::as_address or iris::views::as_raw_ptr
  • We want both iris:::views::XXX(r) and r | iris::views::XXX

Spec

• Plain variable -> std::addressof(v)
• Fancy pointer -> std::to_address(v)
• Raw pointer -> basically returns as-is; for corner cases see below

Notes

• We can't directly use std::to_address for raw pointers because it renders the program ill-formed for function pointers; I believe we do need to accept function pointers.
• We definitely need iris::is_fancy_pointer<T>, check iris::is_fancy_pointer<T> is needed #65.