Speed up twisted.python.context by copying less data

[exarkun]

ContextTracker.callWithContext makes a copy of the current context and update it with the new context data. This makes callWithContext somewhat expensive, even if no code actually loads anything from the context.

Contrariwise, it makes it easy to retrieve keys from the context, which most code doesn't bother to do.

We should reverse these, making callWithContext as cheap as we can, even at the cost of slowing down getContext somewhat.

[exarkun]

(In [31152]) Branching to 'faster-callwithcontext-4960'

[exarkun]

(In [31153]) Make the context stack into a chain that you have to follow, instead of a tower with everything piled on top

refs #4960

[exarkun]

This doesn't seem to change CPython performance measurably, but it speeds up some benchmarks on PyPy by... maybe... 5% or so (eyeball).

[cyli]

 build results are good.

The only concern I have is about the performance cost of exceptions, but this concern may not be valid if getContext most often gets the last context appended to the chain. I'm not sure how getContext is usually used.

Otherwise, looks good to merge.

[exarkun]

(In [31183]) Merge faster-callwithcontext-4960

Author: exarkun, fijal Reviewer: cyli Fixes: #4960

Speed up event dispatch (mostly on PyPy) by reducing the amount of copying involved in the context/logging setup done for each event, at the expense of making key lookup in the context potentially less efficient.