Copy-on-write support

[StrikerX3]

These changes add a new access mode that performs copy-on-write, allowing changes to be made to the data in memory without them being committed to the file system. The new access mode is exposed through the new mio::basic_mmap_cow_sink and its convenience aliases mio::mmap_cow_sink and mio::ummap_cow_sink as well as the shared counterparts, and they can be created with mio::make_mmap_cow_sink(...) just like regular sinks. This implements the feature requested in #42.

One use case is for emulators to load and modify ROM files in memory to decrypt data or apply patches without touching the original ROM file while also benefitting from the memory-mapped file characteristics.

I've confirmed these changes work on Windows 10 and Ubuntu 20.04. I don't have a macOS system, though.

This also fixes the s_2_ws linking issue mentioned in #75 and missing <vector> header on MSVC (#76).