libstd: fix comment in to_str impl of tuple

There is a pointer to #4760, which is a closed issue.  The real issue is
the more general problem described in #4653.  Correct the comment.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
libstd: use fmt! in to_str impl for (one|two)-tuple

The three-tuple uses fmt!, and there's no reason to hand-concatenate
strings.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
auto merge of #6975 : artagnon/rust/tuple-tostr, r=bstrie

[1/2] fixes a couple of comments.
[2/2] uses fmt! instead of hand-concatenating strings in a couple of places.

No functional changes.