Minor changes in std::mem

Fixed a typo in the documentation of std::mem, and refactored a function to use match instead of if.

Also added a FIXME to the benchmarks at the end of the file stating that they should be moved to another place, because they have nothing to do with `mem` (see https://github.com/mozilla/rust/issues/13642)
auto merge of #13643 : aochagavia/rust/pr-2, r=alexcrichton

Fixed a typo in the documentation of std::mem, and refactored a function to use match instead of if.

Also added a FIXME to the benchmarks at the end of the file stating that they should be moved to another place, because they have nothing to do with `mem` (see https://github.com/mozilla/rust/issues/13642)