Support for multi-part frame sending

This update adds support for sending very long frames.

Reception still uses a buffer, because the frame can't be handled until the checksum is verified, however transmission is now virtually unconstrained.

The TX buffer can be very small - perhaps 16 bytes - (must be enough to fit the entire header) and it'll still work fine. Caution: Make sure your LEN field can contain length of the largest payloads!

This is useful for data harvesting applications where TinyFrame in the end device sends large data (perhaps samples from ADC), but doesn't necessarily need to receive anything big. On the other side, the master (PC) can afford to allocate kilobytes of RX buffer without problems.