[v3] Add missing forward_list<>::splice_after and merge overloads fix splice_after taking a range

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

Nico noticed in private email that we weren't implementing LWG 1310, 
thus I proceede to add the required overloads, pretty trivial work 
(suited for 4.7.1 too).

But then, when I got to the testcases I noticed that really almost 
nothing worked as expected for the splice_after overload taking a range: 
memory leaks easily noticeable for existing testcases too; also showing 
up as "disappearing" elements of the destination forward_list; in some 
cases the end of the range improperly handled (thus, as , last], instead 
of last), as must always be). Hey, we had a complexity much lower than 
the expected O(distance(first, last)) per C++11 ;) Thus I'm fixing this 
rather serious issue too. Have a look, tomorrow I mean to commit to 
mainline.

Tested x86_64-linux, normal and debug (and a lot of valgrind too).

Thanks,
Paolo.

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