std: mark two helper functions #[inline].

`str::utf8_char_width` and `char::from_u32` are tiny, which means it's a
big performance hit to call them in a tight loop outside libstd.

src/libstd/str.rs

@@ -1061,6 +1061,7 @@ static UTF8_CHAR_WIDTH: [u8, ..256] = [
 ];
 
 /// Given a first byte, determine how many bytes are in this UTF-8 character
+#[inline]
 pub fn utf8_char_width(b: u8) -> uint {
     return UTF8_CHAR_WIDTH[b] as uint;
 }