Optimize FromIterator by using fold.

If you're curious why this works, see [1]. The benchmark results on
Rust 1.42.0:

simple iterator         time:   [5.3103 ns 5.3130 ns 5.3161 ns]
                        change: [+0.7242% +1.1547% +1.5162%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 12 outliers among 100 measurements (12.00%)
  7 (7.00%) high mild
  5 (5.00%) high severe

chained iterator        time:   [15.553 ns 15.566 ns 15.576 ns]
                        change: [-15.667% -15.456% -15.270%] (p = 0.00 < 0.05)
                        Performance has improved.

[1]: https://medium.com/@veedrac/rust-is-slow-and-i-am-the-cure-32facc0fdcb

Author: meithecatte

enumflags/src/lib.rs

@@ -378,11 +378,7 @@ where
     where 
         I: IntoIterator<Item = B>
     {
-        let mut flags = BitFlags::empty();
-        for flag in it {
-            flags |= flag.into();
-        }
-        flags
+        it.into_iter().fold(BitFlags::empty(), |acc, flag| acc | flag)
     }
 }