py/objstr: Add new mp_obj_new_str_from_cstr() helper function.

There were lots of places where this pattern was duplicated, to convert a
standard C string to a MicroPython string:

    x = mp_obj_new_str(s, strlen(s));

This commit provides a simpler method that removes this code duplication:

    x = mp_obj_new_str_from_cstr(s);

This gives clearer, and probably smaller, code.

Signed-off-by: Jon Foster <jon@jon-foster.co.uk>

py/obj.h

@@ -971,6 +971,7 @@ mp_obj_t mp_obj_new_int_from_str_len(const char **str, size_t len, bool neg, uns
 mp_obj_t mp_obj_new_int_from_ll(long long val); // this must return a multi-precision integer object (or raise an overflow exception)
 mp_obj_t mp_obj_new_int_from_ull(unsigned long long val); // this must return a multi-precision integer object (or raise an overflow exception)
 mp_obj_t mp_obj_new_str(const char *data, size_t len); // will check utf-8 (raises UnicodeError)
+mp_obj_t mp_obj_new_str_from_cstr(const char *str); // // accepts null-terminated string, will check utf-8 (raises UnicodeError)
 mp_obj_t mp_obj_new_str_via_qstr(const char *data, size_t len); // input data must be valid utf-8
 mp_obj_t mp_obj_new_str_from_vstr(vstr_t *vstr); // will check utf-8 (raises UnicodeError)
 #if MICROPY_PY_BUILTINS_STR_UNICODE && MICROPY_PY_BUILTINS_STR_UNICODE_CHECK