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.

tests/unicode/unicode.py

@@ -33,3 +33,17 @@ try:
     int('\u0200')
 except ValueError:
     print('ValueError')
+
+# test invalid UTF-8 string
+try:
+    str(b'ab\xa1', 'utf8')
+except UnicodeError:
+    print('UnicodeError')
+try:
+    str(b'ab\xf8', 'utf8')
+except UnicodeError:
+    print('UnicodeError')
+try:
+    str(bytearray(b'ab\xc0a'), 'utf8')
+except UnicodeError:
+    print('UnicodeError')