This PR:
- enables tests with `import std` usage in CI
- add all tests to build with cmake_subdir_test (because there was only a subset of tests supported)
- fixes tests build with `libc++`
- disables some tests with wchar_t with 'libc++'
* C++ Standard Library based streams now constructed in less cases leading to **better performance**;
* less template instantiations and simpler maintainance;
* support for `volatile` input types was dropped, following the C++ Standard Library trend.
Fedor Osetrov