This patch in effect renames MICROPY_DEBUG_PRINTER_DEST to
MICROPY_DEBUG_PRINTER, moving its default definition from
lib/utils/printf.c to py/mpconfig.h to make it official and documented, and
makes this macro a pointer rather than the actual mp_print_t struct. This
is done to get consistency with MICROPY_ERROR_PRINTER, and provide this
macro for use outside just lib/utils/printf.c.
Ports are updated to use the new macro name.
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