Fix promise data processing

[andrewjaykeller]

Ideally closes #115 by removing promises from all buffer processing functions.

[codecov-io]

Current coverage is 93.94% (diff: 92.94%)

Merging #117 into 1.4.0 will decrease coverage by 0.16%

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[baffo32]

This looks great to me. I think it really increases the clarity (and efficiency) of the code to not use promises for synchronous internal functions. I found a few minor issues which I made line comments for but are not that important. I didn't review the tests, and I don't have an understanding of the packet processing code to really know what is going on in context yet.

One thing is missing: I don't see anywhere that these newly thrown errors are caught and emitted. That's important, so the user of the library can know about them. That might happen in _processBytes or some common wrapping function with code such as

try {
  // do stuff that could cause a throw somewhere inside
} catch(err) {
  this.emit('error', err);
  // do anything needed to clean up after an error, perhaps nothing
}

then the user of the library can listen to the error event to know when something goes wrong.

EDIT: if the error isn't caught somehow it will propagate to the serial library and could mess up its behavior, that would be un-ideal. So even if it isn't emitted it should be caught somewhere without being re-thrown.