ljglibs: Wrap a slew of process control operations

Started with the g_subprocess API, only to discover that it's a too
recent addition to be usable from Howl. Wrapped parts of the g_spawn
API after that.
jit.off for InputStream.read_async
Add howl.dispatch module

For handling coroutine dispatch and resuming.
ljglibs: Add in & output streams and more process stuff
Add copyright notice
Add first version of howl.io.process and friends

Still a work-in-progress, but the raw basics are there.