multi-byte tag support.

length_of_tag: new fun.
encode_tag: new fun.
_TLV1, _TLV: separate tag & flags args.
_OBJID: avoid cython warning.
APPLICATION: made more like CONTEXT.
check_pos, inc_pos, check_has: end-of-stream helpers.
_decode_tag: handle single & multi-byte tags, separate tag & flags.