The Ruby FFI gem defines FFI::Pointer.size, JRuby FFI does not.
The size method is nice to have in case you ever subclass FFI::AutoPointer, and then want to create a memory pointer from that, which needs the size:
ptr = Class.new(FFI::AutoPointer) do
# something clever
# def self.size
FFI::MemoryPointer.new(ptr) # raises error on jruby, since size is missing
Add FFI::Pointer.size to fix #607