merged branch dlsniper/loader-exception-improvement (PR #841)

This PR was squashed before being merged into the master branch (closes #841).

Commits
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08ecb0e Improvements for loader speeds

Discussion
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Improvements for loader speeds

This is something started from #822 .

It attempts to improve the speed of the chain loader and rest of the standard loaders by adding a cache level and a ```hasSource()``` method for them in order gain speed.

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by dlsniper at 2012-09-27T14:56:24Z

Thanks for the feedback, I'll do the changes tonight or during the coming weekend.

As for the utility of the ```::setExtendedErrorHandling()``` function, this saves me about 25% to 40% on the benchkmark Fabien presented in the original ticket.

Thanks!

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by dlsniper at 2012-09-29T11:34:59Z

I've added some caching for the Chain Loader but it's still a WIP. I'll work more on this in the coming hours.

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by dlsniper at 2012-09-29T23:10:10Z

I'm getting better values right now that I had before, standing on:
```
0.09
0.07 ... x 1
0.07 ... x 1
```
I've also noticed that if I run the tests from the browser then I'll have an overall slowdown of about 0.10 which I'm not sure from where it comes from. Any pointers before me digging a bit more into Twigs internals.

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by dlsniper at 2012-10-03T21:31:15Z

@fabpot what's your opinion about this PR?

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by dlsniper at 2012-10-14T18:26:13Z

@Tobion and @fabpot done and done :)

I'm not sure why the tests are failing on PHP 5.4 as I don't have it installed anywhere.

diff --cc lib/Twig/Error.php
Simple merge

diff --cc lib/Twig/Loader/Filesystem.php
Simple merge