py/vstr: Combine vstr_new_size with vstr_new since they are rarely used.

Now there is just one function to allocate a new vstr, namely vstr_new
(in addition to vstr_init etc).  The caller of this function should know
what initial size to allocate for the buffer, or at least have some policy
or config option, instead of leaving it to a default (as it was before).

py/objstringio.c

@@ -150,7 +150,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(stringio___exit___obj, 4, 4, stringio
 STATIC mp_obj_stringio_t *stringio_new(const mp_obj_type_t *type) {
     mp_obj_stringio_t *o = m_new_obj(mp_obj_stringio_t);
     o->base.type = type;
-    o->vstr = vstr_new();
+    o->vstr = vstr_new(16);
     o->pos = 0;
     return o;
 }