std: Disallow bytes 0xC0, 0xC1 (192, 193) in utf-8

Bytes 0xC0, 0xC1 can only be used to start 2-byte codepoint encodings,
that are 'overlong encodings' of codepoints below 128.

The reference given in a comment -- https://tools.ietf.org/html/rfc3629
-- does in fact already exclude these bytes, so no additional comment
should be needed in the code.

Author: blake2-ppc

src/libstd/str.rs

@@ -712,7 +712,7 @@ static UTF8_CHAR_WIDTH: [u8, ..256] = [
 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, // 0x9F
 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, // 0xBF
-2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,
+0,0,2,2,2,2,2,2,2,2,2,2,2,2,2,2,
 2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2, // 0xDF
 3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3, // 0xEF
 4,4,4,4,4,0,0,0,0,0,0,0,0,0,0,0, // 0xFF