From what I could investigate, msgpack doesn't modify char* buffers
obtained from RSTRING_PTR. This means that on Rubinius we don't have to
copy back and forth the buffer to make sure it's also updated on the
Ruby side.
This copying of buffers is a similar problem as the RARRAY_PTR problem,
because it is not safe to expose GC'ed memory on Rubinius to extensions
since it can move due to Rubinius having a moving GC.
Using internals of MRI by using RARRAY_PTR makes it necessary for other
implementations such as Rubinius to continuously copy the structure
returned by RARRAY_PTR back and forth since in Rubinius objects are
layed out differently internally.
Extensions should not depend and use these internal MRI structures if
this is not necessary and when there are API methods that can provide
the same functionality. This makes sure other implementations can also
use the extension without any big problems.
For this reason I also removed the FIXME comment, since that change
would also heavily depend on the internal memory layout of objects on
MRI.
