Lookup idx

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this pull request experiments with a lookup table instead of the inline idx calculations.

--> Works!

tested if it is gaining speed:
in orig. master a call to setRGB(id, r, g, b) resulted in ~49us
after a hardcoded lookup-table it resulted in ~10us
so that is a nice gain!
update: in last test with Optimization activated i got to 9us :-)

TODO:

• generate lookup table at compile time
• try if progmem is fast enough (freeing ram)
• needs c++14 features - blocked by Bump AVR c++ standard to 2014 from 2011 arduino/Arduino#7090

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progmem works fine :-)
results in ~11us for setRGB(id, r, g, b) call

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ok - found the missing information:
for a fully working compile-time generation of the array we need the support of the C++14 feature Relaxing requirements on constexpr functions
but this is first available in gcc 5
that should hopefully come in the near feature: currently blocked by arduino/Arduino#7090

i tried to generate the array with templating magic (you can find a big bunch of ideas and solutions on stackoverflow) - but they work with recursive templating calls...
and the default max recursion depth for the IDE is set at 900. thats not enough.

i have manually copied together a hardcoded lookup table for up to 2400 channels - that is 200 TLC-Chips. i think that enough for the most users ;-)
it bloats the code size ab bit but hey- therefore it gains speed ;-)

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that is it for now :-)

with the last commit i implemented a compile-time generator for the idx lookup table.
its based on this avrfreaks forum post

this compiles fine in the arduino-PR-beta1.9-BUILD-71 version.
you must enable c++14 support in /home/$USER/.arduino15/packages/arduino/hardware/avr/1.6.20/platform.txt:
change -std=gnu++11

compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 ....

to -std=gnu++14

compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++14 ....

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Please support C++17 by default #435
for samd i currently found no such request.