New functions, dynamic buffers

• improve test coverage
• filesize, hard_link_count: return <uintmax_t>-1 on error
• Windows: use wide path internally
• only auto-set CXX_STANDARD if Ffilesystem is top-level project or consuming project less than C++17
• use dynamic memory buffers instead of fixed where possible
• internally, used Filesystem:: namespace for possible future backend selection (e.g. std::filesystem vs boost::filesystem)
• add fs_inode(), fs_st_dev()
• add fs_filesep()
• copy_file() reimplement as RAII
• use HAVE_STATX for Linux and Android - This is like other contemporary projects (Zig, Python, ...)
• add android.cmake toolchain example
• add get_free_memory(), total_sys_memory()
• add has_filename()
• add lexists()