[v3] Add std::is_assignable & co

[Paolo Carlini]

Hi,

tested x86_64-linux, committed to mainline. Many thanks to Daniel for 
the huge collection of is_assignable testcases!

Paolo.

/////////////////////
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: CL_is_assignable
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20110519/96f1b2c0/attachment.ksh>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: patch_is_assignable
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20110519/96f1b2c0/attachment-0001.ksh>