Boost.Filesystem v4 will contain breaking changes from v3 that are required
for better compatibility with C++17 std::filesystem. It will also remove
the deprecated features of v3.
Updated docs to reflect the differences between v3 and v4. Updated tests
to verify both v3 and v4 where the differences are present.
The directory_options enum reflects the same-named enum from C++20. It is now
supported by both directory_iterator and recursive_directory_iterator. In
particular, both iterators now support skip_permission_denied option.
recursive_directory_iterator is now set to end by default on errors, as
required by C++20. An additional directory_options::pop_on_error policy
is added to allow the iterator recover from an error. When this option is
specified and an error occurs, the iterator repeatedly pops the recursion level
until the pop completes successfully or the end state is reached.
recursive_directory_iterator that have standard counterparts (level,
no_push_pending, no_push_request and no_push) are now deprecated and can be
removed by defining BOOST_FILESYSTEM_NO_DEPRECATED. These members will be
removed in a future release.
Docs and tests updated accordingly. Also, in docs reconstructed release history
for the past releases from Boost release notes.
Fixes https://github.com/boostorg/filesystem/issues/112
Fixes https://github.com/boostorg/filesystem/issues/113
