I2C_SET was quite inflexible, it used too long delays as well as some
unnecessary delays. The new building blocks are iicbb_clockin and
iicbb_clockout. The former sets SDA and starts the high period of SCL, the
latter executes the low period of SCL. What happens during the high phase
depends on the operation. For writes we just hold both lines, for reads we
poll SDA. S, Sr and P change SDA in the middle of the high period.

Also, the calculation of udelay has been updated, so that the resulting
period more closely corresponds the requested bus frequency.
There is a new knob, io_delay, that allows to further adjust udelay based on
the estimated latency of pin toggling operations.

Finally, I slightly changed debug tracing and added error indicators to it.
The debug prints are compiled in but disabled by default.
This can be of use if there is any fallout from this change.

Some ideas for further improvements:

• add a function for sub-microsecond delays (e.g., in units of 1/10th of a microsecond) and use it for more precise timing of short delays;
• account for the actual time spent in the pin I/O.

Reading temperature and humidity from HTU21 in the bus hold mode:

<<w80+ we3+ <w81+ .....r6d+ rac+ r94- >>
<<w80+ we5+ <w81+ .............r47+ re2+ r84- >>

where '<<' is S, '<' is Sr, '>>' is P, '.' is one millisecond of clock
stretching by the slave.

Reading temperature and humidity in the no-hold mode:

<<w80+ wf3+ >>
<<w81- >>
<<w81+ r6d+ r54+ raf- >>
<<w80+ wf5+ >>
<<w81- >>
<<w81+ r48+ r4e+ r9c- >>

where '+' is Ack and '-' is NoAck.
We see that first read attempts are not acknowledged.