Move html case up since it is the most likely to be hit
Use plain string comparison which is twice as fast
Avoid strtolower call at every twig_escape_filter call
Add UTF-8 in uppercase since it is so common
Use uppercased charsets consistently (as in js/css/html_attr strategies)
Avoid unnecessary is_object call
Make happy path (strings) faster by avoiding the cast, same perfs for the other cases
merged branch Seldaek/optim (PR #1077)

This PR was merged into the master branch.

Discussion
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Some more optimizations

Please look at commits one by one. Overall this makes the twig_escape_filter almost 20% faster here for happy path (escaping string for html with UTF-8) on 1000 calls. Still pretty darn slow, but faster.

Commits
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d886b12 Make happy path (strings) faster by avoiding the cast, same perfs for the other cases
97bf5e5 Avoid unnecessary is_object call
c1c0dcc Use uppercased charsets consistently (as in js/css/html_attr strategies)
a23fa62 Add UTF-8 in uppercase since it is so common
c891c53 Avoid strtolower call at every twig_escape_filter call
61f2b2a Use plain string comparison which is twice as fast
d4eec01 Move html case up since it is the most likely to be hit