Read and write JSONLab/JData flavored optimized N-D array header

[fangq]

see https://github.com/ubjson/universal-binary-json/issues/61#issuecomment-492445324

Fixed a few gcc warnings, added the missing ubjw_begin_ndarray() function.

valgrind now gives me a lost buffer inside priv_ubjr_read_raw_array() which I modified, I suspect it is related to this line, not sure if that call releases the memory associated with the temp variable.