i2c: concentrate sample number and value getting in main loop

i2c: concentrate sample number and value getting in main loop

It's unfortunate how the symbol / bit value handlers of the I2C decoder
keep redundantly accessing the .samplenum property. Ideally they should
just get an ss, es, value tuple, while the determination of these params
should be kept in the .decode() main loop.

Prepare the internal implementation to that approach, but enforce an
absolutely backwards compatible behaviour for now. This was verified by
the test suite. The data bit handler still keeps updating previous bits'
end positions when another bit is seen. Which assumes back to back bits
which strictly speaking does not match the protocol's definition. The
unfortunate application of the second last bit time's width to the last
bit time and the ACK slot is kept as well. And the code path needed to
be kept within the bit handler, because the second last bit's width only
becomes available when the last bit _was_ handled. Which means that the
main loop cannot provide a useful es value which matches the previous
implementation's behaviour.