wcp, a faster version of cp with Linux async I/O

From the link:

TLDR: I made a super fast (much faster than cp) command line copy program, with a nice progress bar. The implementation dives into some interesting bits of low level async I/O on linux.

wcp is a file copying experiment I've been working on for the past while. Really, it's an excuse to try out io_uring, a relatively new asynchronous I/O system on linux.