testcases: Add config.h.

There's nothing special about this config.h, it's just the one I
happened to use for first profiling investigations. To allow
everybody else to do the very same profiling runs, I add it here.

Doing profiling isn't too complicated:

  mv config.h config.h.backup
  ln -s testcases/config.h.Profiling config.h
  git checkout -b work
  git cherry-pick simulavr # add tweaks convenient for simulation runs
  make
  cd testcases
  ./run-in-simulavr.sh short-moves.gcode smooth-curves.gcode triangle-odd.gcode

After being done you can restore your config.h and delete this work branch.

Currently, performance is as following (with convenience commit applied):

    SIZES             ATmega...  '168    '328(P)    '644(P)    '1280
    FLASH : 20270 bytes          142%       66%        32%       16%
    RAM   :  2302 bytes          225%      113%        57%       29%
    EEPROM:    32 bytes            4%        2%         2%        1%

short-moves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 888.
Sum of all LED on time: 279945 clock cycles.
LED on time minimum: 306 clock cycles.
LED on time maximum: 722 clock cycles.
LED on time average: 315.253 clock cycles.

smooth-curves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 9124.
Sum of all LED on time: 3297806 clock cycles.
LED on time minimum: 311 clock cycles.
LED on time maximum: 712 clock cycles.
LED on time average: 361.443 clock cycles.

triangle-odd.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 1636.
Sum of all LED on time: 546946 clock cycles.
LED on time minimum: 306 clock cycles.
LED on time maximum: 712 clock cycles.
LED on time average: 334.319 clock cycles.
DDA: Move axis calculations into loops, part 6a.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.

Part 6a is putting stuff inside the step interrupt into a loop,
too. do_step() is put into the "tidiest" place. Binary size goes
down a remarkable 374 bytes, but stepping performance suffers by
almost 30%.

Traumflug's performance measurements:

    SIZES             ATmega...  '168    '328(P)    '644(P)    '1280
    FLASH : 19908 bytes          139%       65%        32%       16%
    RAM   :  2302 bytes          225%      113%        57%       29%
    EEPROM:    32 bytes            4%        2%         2%        1%

short-moves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 888.
Sum of all LED on time: 354537 clock cycles.
LED on time minimum: 390 clock cycles.
LED on time maximum: 806 clock cycles.
LED on time average: 399.253 clock cycles.

smooth-curves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 9124.
Sum of all LED on time: 4268896 clock cycles.
LED on time minimum: 395 clock cycles.
LED on time maximum: 807 clock cycles.
LED on time average: 467.875 clock cycles.

triangle-odd.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 1636.
Sum of all LED on time: 706846 clock cycles.
LED on time minimum: 390 clock cycles.
LED on time maximum: 807 clock cycles.
LED on time average: 432.057 clock cycles.
DDA: Move axis calculations into loops, part 6b.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.

Part 6b moves do_step() from the "tidiest" place into where it's
currently used, dda.c. Binary size goes down another 34 bytes, to
a total savings of 408 bytes and performance is much better, but
still 16% lower than without using loops:

    SIZES             ATmega...  '168    '328(P)    '644(P)    '1280
    FLASH : 19874 bytes          139%       65%        32%       16%
    RAM   :  2302 bytes          225%      113%        57%       29%
    EEPROM:    32 bytes            4%        2%         2%        1%

short-moves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 888.
Sum of all LED on time: 320000 clock cycles.
LED on time minimum: 351 clock cycles.
LED on time maximum: 772 clock cycles.
LED on time average: 360.36 clock cycles.

smooth-curves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 9124.
Sum of all LED on time: 3875874 clock cycles.
LED on time minimum: 356 clock cycles.
LED on time maximum: 773 clock cycles.
LED on time average: 424.8 clock cycles.

triangle-odd.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 1636.
Sum of all LED on time: 640357 clock cycles.
LED on time minimum: 351 clock cycles.
LED on time maximum: 773 clock cycles.
LED on time average: 391.416 clock cycles.
DDA: Move axis calculations into loops, part 6c.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.

Part 6c removes do_step(), but still tries to keep a loop. This
about the maximum of performance I (Traumflug) can think of.
Binary size is as good as with the former attempt, but performance
is actually pretty bad, 45% worse than without looping:

    SIZES             ATmega...  '168    '328(P)    '644(P)    '1280
    FLASH : 19876 bytes          139%       65%        32%       16%
    RAM   :  2302 bytes          225%      113%        57%       29%
    EEPROM:    32 bytes            4%        2%         2%        1%

short-moves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 888.
Sum of all LED on time: 406041 clock cycles.
LED on time minimum: 448 clock cycles.
LED on time maximum: 864 clock cycles.
LED on time average: 457.253 clock cycles.

smooth-curves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 9124.
Sum of all LED on time: 4791132 clock cycles.
LED on time minimum: 453 clock cycles.
LED on time maximum: 867 clock cycles.
LED on time average: 525.113 clock cycles.

triangle-odd.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 1636.
Sum of all LED on time: 800586 clock cycles.
LED on time minimum: 448 clock cycles.
LED on time maximum: 867 clock cycles.
LED on time average: 489.356 clock cycles.
DDA: Revert move axis calculations into loops, part 6a-c.

Sad but true, this experiment didn't work out. Performance loss
due to looping in dda_step() is still at least 16% with the best
algorithm found.