msgpack: allow to pass 'struct ibuf *' into encode()

Before the patch msgpack Lua module provided a method encode()
able to take a custom buffer to encode into. But it should be of
type 'struct ibuf', what made it impossible to use
buffer.IBUF_SHARED as a buffer, because its type is
'struct ibuf *'. Strangely, but FFI can't convert these types
automatically.

This commit allows to use 'struct ibuf *' as well, and moves this
functionality into a function in utils.h. Now both msgpack and
merger modules can use ibuf directly and by pointer.