perfrecord-mach-ipc-rendzvous

This crate has some code that allows establishing two-way mach communication between a parent process and a child process.

This code was originally written by pcwalton for ipc-channel. I needed some extra functionality to be able to send raw ports, namely mach_task_self(), so I forked the code. I may also remove large pieces of functionality that I don't need, so that the size of the perfrecord-preload library gets reduced.

This is a separate crate, rather than just a mod inside perfrecord, so that it can also be used by perfrecord-preload.