py/objstr: Add check for valid UTF-8 when making a str from bytes.

This patch adds a function utf8_check() to check for a valid UTF-8 encoded
string, and calls it when constructing a str from raw bytes.  The feature
is selectable at compile time via MICROPY_PY_BUILTINS_STR_UNICODE_CHECK and
is enabled if unicode is enabled.  It costs about 110 bytes on Thumb-2, 150
bytes on Xtensa and 170 bytes on x86-64.

py/unicode.h

@@ -30,5 +30,6 @@
 #include "py/misc.h"
 
 mp_uint_t utf8_ptr_to_index(const byte *s, const byte *ptr);
+bool utf8_check(const byte *p, size_t len);
 
 #endif // MICROPY_INCLUDED_PY_UNICODE_H