Implement the K-distribution as an extension.

libstdc++-v3/ChangeLog

libstdc++-v3/include/ext/random

libstdc++-v3/include/ext/random.tcc

...++-v3/testsuite/ext/random/k_distribution/cons/default.cc

...dc++-v3/testsuite/ext/random/k_distribution/cons/parms.cc

...v3/testsuite/ext/random/k_distribution/operators/equal.cc

.../testsuite/ext/random/k_distribution/operators/inequal.cc

...estsuite/ext/random/k_distribution/operators/serialize.cc

...m/k_distribution/requirements/explicit_instantiation/1.cc

...tsuite/ext/random/k_distribution/requirements/typedefs.cc