use const modifier for return values of hb_{direction,language}_to_string
use const modifier for return values of hb_{direction,language}_to_string

mutes compiler warning 'assignment discards const qualifier from pointer...'

also see:
http://cython.readthedocs.io/en/latest/src/tutorial/strings.html#dealing-with-const

harfbuzz/uharfbuzz,harfbuzz/uharfbuzz