Use Cow in Token::String.

`Printer::word` takes a `&str` and converts it into a `String`, which
causes an allocation. But that allocation is rarely necessary, because
`&str` is almost always a `&'static str` or a `String` that won't be
used again.

This commit changes `Token::String` so it holds a `Cow<'static, str>`
instead of a `String`, which avoids a lot of allocations.

src/libsyntax/parse/parser.rs

@@ -2795,7 +2795,7 @@ impl<'a> Parser<'a> {
                             s.print_usize(float.trunc() as usize)?;
                             s.pclose()?;
                             s.s.word(".")?;
-                            s.s.word(fstr.splitn(2, ".").last().unwrap())
+                            s.s.word(fstr.splitn(2, ".").last().unwrap().to_string())
                         });
                         err.span_suggestion_with_applicability(
                             lo.to(self.prev_span),