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.
diff --git a/py/objfloat.c b/py/objfloat.c
index b94caf3..eb59cc5 100644
--- a/py/objfloat.c
+++ b/py/objfloat.c
@@ -42,24 +42,24 @@
 #include "py/formatfloat.h"
 #endif
 
-STATIC void float_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o_in, mp_print_kind_t kind) {
+STATIC void float_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t kind) {
     (void)kind;
     mp_obj_float_t *o = o_in;
 #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
     char buf[16];
     mp_format_float(o->value, buf, sizeof(buf), 'g', 7, '\0');
-    print(env, "%s", buf);
+    mp_print_str(print, buf);
     if (strchr(buf, '.') == NULL && strchr(buf, 'e') == NULL) {
         // Python floats always have decimal point
-        print(env, ".0");
+        mp_print_str(print, ".0");
     }
 #else
     char buf[32];
     sprintf(buf, "%.16g", (double) o->value);
-    print(env, buf);
+    mp_print_str(print, buf);
     if (strchr(buf, '.') == NULL && strchr(buf, 'e') == NULL) {
         // Python floats always have decimal point
-        print(env, ".0");
+        mp_print_str(print, ".0");
     }
 #endif
 }