- Add the "VCPKG_DEVELOPMENT_WARNINGS" flag
	- setting "WERROR" will also set this flag
	- This flag is set by default
	- on GCC/clang, this will pass '-Wall -Wextra -Wpedantic -Werror'
	- on GCC, this will additionally pass '-Wmissing-declarations'
	- on clang, this will additionally pass '-Wmissing-prototypes'
	- on MSVC, this will pass '-W4 -WX'
- On Visual Studio 2017 and later, pass '-permissive-'
- Change the source for fallout of these changes
- add `format` subcommand
	- formats all C++ source and header files using clang-format
- move `include/vcpkg-test/catch.h` to `include/catch2/catch.hpp`
- pass CONFIGURE_DEPENDS to file(GLOB)

I added benchmarks to measure how fast the parallel remove_all code was
-- it turns out, about 3x slower than stdfs::remove_all. Since this was
the case, I removed all of the parallelism and rewrote it serially, and
ended up about 30% faster than stdfs::remove_all (in addition to
supporting symlinks).

In addition, I did the following three orthogonal changes:
  - simplified the work queue, basing it on Billy O'Neal's idea
  - Fix warnings on older versions of compilers in tests, by splitting
    the pragmas out of pch.h.
  - Ran clang-format on some files

In fixing up remove_all, the following changes were made:
  - On Windows, regular symlinks and directory symlinks are distinct;
    as an example, to remove directory symlinks (and junctions, for that
    matter), one must use RemoveDirectory. Only on Windows, I added new
    `file_type` and `file_status` types, with `file_type` including a new
    `directory_symlink` enumerator, and `file_status` being exactly the
    same as the old one except using the new `file_type`. On Unix, I
    didn't make that change since they don't make a distinction.
  - I added new `symlink_status` and `status` functions which use the
    new `file_status` on Windows.
  - I made `Filesystem::exists` call `fs::exists(status(p))`, as opposed
    to the old version which called `stdfs::exists` directly.
  - Added benchmarks to `vcpkg-test/files.cpp`. They test the
    performance of `remove_all` on small directories (~20 files), with
    symlinks and without, and on large directories (~2000 files), with
    symlinks and without.