I2C module for picoLisp:

* Fix compiler warnings.
* Cleanup comments.
i2c_write() for picoLisp.
i2c module: change function names.

Module function are now called
'plisp_i2c_*()'. i2c_*() already
declared in avr32/i2c.h.
Compile i2c support by default for Mizar32.