Memory manager + two new architectur files

Memory manager:
changed pico_zalloc to PICO_ZALLOC and pico_free to PICO_FREE in the whole stack...

Unit tests can be run by doing:
make units_mm MEMORY_MANAGER=1
./build/test/units_mm

New architectures:
for the two new architectures ( AVR and arm STR9 ), the Makefile has been changed and pico_avr.h and pico_str9.h are added to the arch folder