[CogManager, Utils] Handle missing cogs correctly, add some helpful algorithms (#1989)

* Handle missing cogs correctly, add some helpful algorithms

For cog loading, only show "cog not found" if the module in question was the one
that failed to import. ImportErrors within cogs will show an error as they should.

- deduplicator, benchmarked to be the fastest
- bounded gather and bounded async as_completed
- tests for all additions

* Requested changes + wrap as_completed instead

So I went source diving and realized as_completed works the way I want it to,
and I don't need to reinvent the wheel for cancelling tasks that remain
if the generator is `break`ed out of. So there's that.

docs/framework_utils.rst

@@ -4,6 +4,12 @@
 Utility Functions
 =================
 
+General Utility
+===============
+
+.. automodule:: redbot.core.utils
+    :members: deduplicate_iterables, bounded_gather, bounded_gather_iter
+
 Chat Formatting
 ===============