Do not use nilable Pointers

[HertzDevil]

These uses do not distinguish between a null pointer and nil, and dropping the Nil reduces the space used (every IO::Buffered now occupies 16 less bytes on 64-bit systems).

The same effect could also be achieved by supporting nilable pointers at the ABI level, similar to how Proc? has the same binary representation as Proc. That approach is not without its own problems:

proc = Proc(Nil).new(Pointer(Void).null, Pointer(Void).null)
proc                  # => #<Proc(Nil):0x0>
!!proc                # => true
proc.as(Proc(Nil)?)   # => nil
!!proc.as(Proc(Nil)?) # => false