bug #1258 Fix twig_first and twig_last filters for UTF8 multibyte strings (chEbba)

This PR was squashed before being merged into the master branch (closes #1258).

Discussion
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Fix twig_first and twig_last filters for UTF8 multibyte strings

After string is sliced we already have the right result with a first/last letter for UTF8 string.

Commits
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6003ae4 Fix twig_first and twig_last filters for UTF8 multibyte strings

diff --cc lib/Twig/Extension/Core.php
Simple merge