Yet another bit closer to integer-only math.

No performance improvements, this time.

All this move towards integers has brought a small, but noticeable
change in results: large deviations for 2..8 and 0 or +/-.25 for 28+
Likely some rounding errors.
Use a polynomal and performance is back.

LED on time minimum: 1592 clock cycles.
LED on time maximum: 1778 clock cycles.
LED on time average: 1621.56 clock cycles.

Precision isn't that bad, either ...
These polynomals found by linear regression are very fast and nice,
but finding their coefficients is at least as complex as making
thermistor tables.

Thermistor manufacturers provide no such coefficients, only R0,
T0, Beta. Accordingly, turn back to find an algorithm
logarithm.

Nice thing: This algorithm the HP-35 pocket calculator used could
be implemented successfully. Right now, a nice demonstration is
coded.
Other than HP-35, choose 2 as target number. In theory, this
should be faster than target 10, but actually, it's worse:

With hp_35_log():
LED on time minimum: 1949 clock cycles,
LED on time maximum: 22478 clock cycles,
LED on time average: 14977,3 clock cycles,

With teacup_log():
LED on time minimum: 1949 clock cycles,
LED on time maximum: 24425 clock cycles,
LED on time average: 16669,5 clock cycles,
teacup_log() collect the result as 8.24 fixed point directly.

While this replaces additions/substractions, only, performance
is quite a bit better already:

LED on time minimum: 1937 clock cycles,
LED on time maximum: 16446 clock cycles,
LED on time average: 11528,6 clock cycles,