The resulting code is actually simpler, and more powerful at the
same time

This clarifies the code. It also reduces the code size
by another 1000 bytes or so.

This reduces the code size by about 500 bytes (e.g. TypeD_REV example
for me went from 5242 to 4738 bytes, a saving of about 10%)

* move the PROTOCOL3_ #defines from .h to .c and turn them into enums
  (debugging enums is easier than #defines)
* add sync factor constants for protocols 1 and 2
* use the PROTOCOL* constants in more places

Fixes some typos, and wraps it for readability

These help to catch bugs, and also can sometimes help the compiler
optimize the code

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- receive without callback now

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