Fixed some sign comparison warnings.
Added utility functions to fr::Packet's for:
Getting read cursor position.
Relative read cursor seeking.
Asserting that data remains in the packet
Getting the amount of data remaining
Getting the size of the packet
Replaced instances of resize&memcpy with append, which gives a noticeable performance boost.
fr::Packet::operator<<(const char *str) no longer converts str into an std::string before adding it, removing an unneeded copy.
fr::Packet::clear no longer calls erase, should result in more of the internal buffer remaining allocated.
Framing for std::vector's has been changed from a uint64_t to a uint32_t (breaking change for packet framing!)
