Special case inlineCallbacks for case where Deferred already has value

[itamarst]

The value of a Deferred can be extracted without addCallbacks. Having inlineCallbacks do this should speed it up. Obviously this breaks abstraction, but (only!) within context of t.i.defer that's probably fine.

Benchmarks I sent to jknight separately demonstrate this change speeds up this case dramatically, and slightly speeds up the other case (with a buggy implementation, but still).

[Jean-Paul Calderone]

The benchmarks should be added to Twisted as well, of course. I think it would be good if they were actually the main focus of this ticket. This sounds like a good optimization, but specific optimization should be driven by the benchmarks, not just justified by them.

[itamarst]

Benchmarks for inlineCallbacks and friends

[itamarst]

Cleaned up benchmarks are attached. They include the (buggy) optimization to inlineCallbacks.