Remove using-declarations that add std names to __gnu_cxx
These using-declarations appear to have been added for simplicity when
moving the non-standard extensions from namespace std to namespace
__gnu_cxx. Dumping all these names into namespace __gnu_cxx allows
unportable uses like __gnu_cxx::size_t and __gnu_cxx::pair, which serve
no useful purpose.
This patch removes most of the using-declarations from namespace scope,
then either qualifies names as needed or adds using-declarations at
block scope or typedefs at class scope.