- Changed declaration of SAUL classes (while keeping the format)
- Termed "class of SAUL classes" category of SAUL classes in the doc to
avoid confusion. (E.g. a SAUL class will now be in category sensor or
actuator.)
- Separate enums for SAUL category and intra-category ID. Numbers will now
but auto-assigned by the compiler
- Use constant look-up tables for stringification (one table per SAUL category).
==> This saves 512B in .data section of AVR
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