Big refactoring step.

- Moves a lot of stuff from vibe.core.core to vibe.core.task
- Introduces TaskScheduler to unify the scheduling process
- Refines how tasks are scheduled and processed (can push to the front of the task queue and uses a marker task to keep track of the spot up to which to process)
- Start to add proper support for task interrupts and timeouts by properly cancelling in-flight async operations
- Work on ManualEvent - still not functional for the shared case
- Implement proper IP address parsing in NetworkAddress

source/vibe/core/log.d

@@ -859,6 +859,7 @@ private struct LogOutputRange {
 
 	void put(dchar ch)
 	{
+		static import std.utf;
 		if (ch < 128) put(cast(char)ch);
 		else {
 			char[4] buf;