[patch] Add std::bool_constant for C++17

[Jonathan Wakely]

This implements
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4389.html
which was voted into the working paper in Lenexa. Like the last patch,
this has a TODO placeholder for the feature-test macro.

It also updates the manual to document the status of all the library
papers from Lenexa.

Tested powerpc64le-linux, committed to trunk.

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