Remove std::size_t from all header files.

[bmunkholm]

Use:
using std::size_t
using std::time_t

[kspangsege]

Good looks is less important that correctness.

The problem with not using std::size_t and std::time_t in headers, is that some compilers (including GCC) are lenient in this regard, so if we specify 'using std::size_t' in some header, but forget to include that header in some other header, then there is an error, but the error is probably not reported, and the error is hard to spot, when you are not looking for it.

One solution might be to require that every header, that use size_t, must themselves specify 'using std::size_t'. This can be checked by a script.