This is akin to channels in Go, except:
 * implemented in userland
 * they are lock-free and thread-safe
 * they integrate with the userland event loop

The self hosted compiler is changed to use a channel for events,
and made to stay alive, watching files and performing builds when
things change, however the main.zig file exits after 1 build.

Note that nothing is actually built yet, it just parses the input
and then declares that the build succeeded.

Next items to do:
 * add windows and macos support for std.event.Loop
 * improve the event loop stop() operation
 * make the event loop multiplex coroutines onto kernel threads
 * watch source file for updates, and provide AST diffs
   (at least list the top level declaration changes)
 * top level declaration analysis