Draft: Remove deprecated system headers
Many system headers like e.g. assert.h are deprecated in C++14. One should use
corresponding C++ headers like e.g. cassert. With this commit all deprecated
headers are excahnged by the proper C++ versions.
A list of these headers can be found at
https://clang.llvm.org/extra/clang-tidy/checks/modernize/deprecated-headers.html