Libuv Asys Implementation

[Aidan63]

WIP Libuv based asys api implementation. Trying to hide the libuv-ness behind a generic asys header so someone could provide an alternative asys implementation for platforms which might not support libuv but have their own. Also going to add a "NotImplementedException" asys implementation for non desktop platforms.

I've tried a base object approach instead of differing implementations behind #ifdef checks so typed objects can be passed around instead of Dynamic. I'm also using some c++ 14 features (make_unique and nested namespaces), not sure if there's a c++ version that hxcpp tries to target. I'm not married to any of this so I'm happy to change any of it.

• Event Loop
• File
• Directory
• File System
• Net
  • Dns
  • Ip
  • Socket
  • Server
  • UdpSocket
  • SecureSocket
  • SecureServer
• Processes
  • stdio pipe to file paths.
• Signals
• Streams

Other TODO:

• Ensure all uv_handle_t's are cleaned up when things fail (look into custom unique_ptr's to automate this)
• Move some stuff out of C++ and into haxe (I would like to move all the queuing stuff into haxe, easier than faffing around with state in a C API).
• Double check all GC and heap object cleanup
• Change all uses of Dynamic to typed callables (depends on [cpp] Generate Typed Functions haxe#11151)
• Reduce dependency to C++ 11.
• Revive branch I experimented on a while ago to mark hx::Throw as no-return.