py: Overhaul and simplify printf/pfenv mechanism.
Previous to this patch the printing mechanism was a bit of a tangled mess. This patch attempts to consolidate printing into one interface. All (non-debug) printing now uses the mp_print* family of functions, mainly mp_printf. All these functions take an mp_print_t structure as their first argument, and this structure defines the printing backend through the "print_strn" function of said structure. Printing from the uPy core can reach the platform-defined print code via two paths: either through mp_sys_stdout_obj (defined pert port) in conjunction with mp_stream_write; or through the mp_plat_print structure which uses the MP_PLAT_PRINT_STRN macro to define how string are printed on the platform. The former is only used when MICROPY_PY_IO is defined. With this new scheme printing is generally more efficient (less layers to go through, less arguments to pass), and, given an mp_print_t* structure, one can call mp_print_str for efficiency instead of mp_printf("%s", ...). Code size is also reduced by around 200 bytes on Thumb2 archs.