merged branch arnaud-lb/twig-escape-filter-opt (PR #537)

Commits
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7b8d476 optimized twig_escape_filter

Discussion
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Optimized twig_escape_filter

This optimizes the part of twig_escape_filter that checks if the charset is supported by htmlspecialchars.

This uses a static variable to avoid initializing the charsets array each time the function is called; and does a hash lookup instead of array search (saves a function call too).

The optimized version takes 66% less time that the original version on html escaping :) (no difference on js escaping).