So MemoryPointer is like calling malloc and returning a pointer to
it.

I had tried just sending address to Pointer.new in hopes that it could
handle an Int as well as an existing pointer. But it doesn't appear to
work.

>> require 'ffi'

=> true

>> address = FFI::Pointer.new(5)

=> #<Native Pointer address=0x5>

>> p = FFI::Pointer.new(address)

TypeError: can't convert FFI::Pointer into Integer
        from (irb):4:in `initialize'
        from (irb):4:in `new'
        from (irb):4

>> address = FFI::MemoryPointer :pointer

NoMethodError: undefined method `MemoryPointer' for FFI:Module
        from (irb):5

>> address = FFI::MemoryPointer.new :pointer

=> #<MemoryPointer address=0x155f140 size=8>

>> p = FFI::Pointer.new(address)

TypeError: can't convert FFI::MemoryPointer into Integer
        from (irb):7:in `initialize'
        from (irb):7:in `new'
        from (irb):7