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.

py/objset.c

@@ -79,7 +79,7 @@ STATIC void check_set(mp_obj_t o) {
     }
 }
 
-STATIC void set_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
+STATIC void set_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
     (void)kind;
     mp_obj_set_t *self = self_in;
     #if MICROPY_PY_BUILTINS_FROZENSET
@@ -88,37 +88,36 @@ STATIC void set_print(void (*print)(void *env, const char *fmt, ...), void *env,
     if (self->set.used == 0) {
         #if MICROPY_PY_BUILTINS_FROZENSET
         if (is_frozen) {
-            print(env, "frozen");
+            mp_print_str(print, "frozen");
         }
         #endif
-        print(env, "set()");
+        mp_print_str(print, "set()");
         return;
     }
     bool first = true;
     #if MICROPY_PY_BUILTINS_FROZENSET
     if (is_frozen) {
-        print(env, "frozenset(");
+        mp_print_str(print, "frozenset(");
     }
     #endif
-    print(env, "{");
+    mp_print_str(print, "{");
     for (mp_uint_t i = 0; i < self->set.alloc; i++) {
         if (MP_SET_SLOT_IS_FILLED(&self->set, i)) {
             if (!first) {
-                print(env, ", ");
+                mp_print_str(print, ", ");
             }
             first = false;
-            mp_obj_print_helper(print, env, self->set.table[i], PRINT_REPR);
+            mp_obj_print_helper(print, self->set.table[i], PRINT_REPR);
         }
     }
-    print(env, "}");
+    mp_print_str(print, "}");
     #if MICROPY_PY_BUILTINS_FROZENSET
     if (is_frozen) {
-        print(env, ")");
+        mp_print_str(print, ")");
     }
     #endif
 }
 
-
 STATIC mp_obj_t set_make_new(mp_obj_t type_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
     mp_arg_check_num(n_args, n_kw, 0, 1, false);