[Patch] Implementation of n3793 <experimental/optional>

[Luc Danton]

Hello everyone,

I have completed the copyright assignment process. I originally submitted a
candidate patch to implement <optional> at the end of July
(http://gcc.gnu.org/ml/libstdc++/2013-07/msg00137.html). This was 
against n3672,
i.e. revision 4 of the optional proposal. As it so happens, n3793 was 
published
Thursday as a 5th revision.

The big change between the two is that optional<T> now lives inside header
<experimental/optional> and namespace std::experimental. The patch I'm
submitting today implements that change, and most of the minor tweaks of 
n3793,
but not the new equality and relational operators. I'll provide those in a
separate patch.

Important things that aren't quite correct and needs reviewing/fixing:
  - include/Makefile.am, which I've modified due to putting the header in
    include/experimental/optional -- tests do compile and pass but I'm 
afraid
    I've otherwise botched it
  - include/Makefile.in needs to be regenerated

Not as important:
  - changes to testsuite/libstdc++-dg/conformance.exp could use a 
review, because
    I've put the tests in testsuite/experimental/optional/
  - the ABI-related matters of bad_optional_access and
    __throw_bad_optional_access will need to be settled somehow (introduce
    src/experimental/optional.cc and
    src/experimental/compatibility_functexcept.cc? I can't really say), 
and until
    then I have left the associated function definitions inline in the 
header --
    they are marked by the only two XXX items of the file

There are still three tests that unexpectly fail, which prompted me to 
file a bug
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58777).

Many thanks to all those that helped me.
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