py/mpprint.h: Add MP_PRN() macro.

py/mpprint.h: Rename MP_PRN to MP_DEBUG_PRINT.

Move MP_DEBUG_PRINT_XXX constants upper to the #if MICROPY_INCLUDED_PY_MPPRINT_H/#endif block.

Co-Authored-By: Andrew Leech <3318786+andrewleech@users.noreply.github.com>

py/mpprint.h: Fix typo.

py/mpprint.h: Move HOWTO  upper.

Add gaps between MP_DEBUG_PRINT_XXX levels.
Move `How to use:` upper before #define MP_DEBUG_PRINT().
Print level names instead of the numbers.

Signed-off-by: Ihor Nehrutsa <IhorNehrutsa@gmail.com>