Add memory debugging code

We recently ran out of SRAM several times and
this results in pretty funny behaviors.

If you enable the new define, you get the amount
of free RAM printed on the serial output.

Also add an own copy of the F() macro to place
strings into flash instead of flash AND program memory.
Put serial strings into flash memory

Normally strings are put into flash memory
and program memory. If we put them in flash memory,
the CPU needs three instead of two cycles to access them.

This gives 100 bytes more RAM instantly and should
help our current out-of-memory situations.
Adapt path in program help