Support building on MSVC (#632)

Support building on MSVC (#632)

With these changes, the project builds with Visual Studio 2019, unit
tests pass and it works correctly with mingw gcc.

NOTE: The very latest version of Visual Studio 2019 is required, because
there was just a necessary fix for template arguments.

Tested building and running unit tests on Windows+MSVC, Windows+MinGW,
Linux and macOS.

- Enable `ZSTD_FROM_INTERNET` by default for MSVC when not using vcpkg
  or conan.

- Add include tests for some standard UNIX headers not available on
  MSVC.

- Add necessary MSVC compiler flags.

- In `Args::from_gcc_atfile()` iterate over the string via `c_str()`
  instead of `cbegin()`, the MSVC string character iterator does not
  include the ending null byte.

- Misc. minor cmake fix-ups.

- Add some headers that are not implicitly included from other headers
  like `<algorithm>`, `<ios>`, `<cstdint>` and `<cstdarg>` in some
  places, gcc does this but MSVC does not.

- Add `std::filesystem` version of `Util::traverse()` when dirent.h is
  not available, which is preferred for performance reasons.

- Add implementations of the following functions that are not available
  in MSVC in Win32Util.cpp: `gettimeofday()`, `localtime_r()`,
  `asprintf()`.

- Add Windows implementation of `getopt_long()` from
  https://www.codeproject.com/Articles/157001/Full-getopt-Port-for-Unicode-and-Multibyte-Microso
  to third_party/win32.

- Add some compatibility typedefs, constants and macros to the `_WIN32`
  section of system.hpp, as well as the prototypes for the functions
  added to Win32Util.cpp.

- Fix up unit tests expecting '/' separated paths to expect paths
  delimited by `DIR_DELIM_CH`.

- Invoke test/run with bash from cmake, necessary on msys2+mingw64, many
  fail, there is more work to do here.

- Set the warning level to `/W4` and silence all the uninteresting
  warning types. Compiles with no warnings now.

- Switch to using standard C++ attributes `[[nodiscard]]` and
  `[[maybe_unused]]` and define macros for gcc for their equivalents.

- `#define DOCTEST_CONFIG_USE_STD_HEADERS` for MSVC only, because it
  requires explicitly including `<ostream>`.

- Add vim files to .gitignore.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>