Update slice.rst
minor #1422 Update slice.rst (1emming)

This PR was merged into the 1.16-dev branch.

Discussion
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Update slice.rst

Please state that Twig `slice` is  multi-byte safe since it uses `mb_substr` :)

Commits
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87165e7 Update slice.rst
added the scaper class for easy autoloading
added composer json
added a test for the escaper
removed backslash on exception catch
newlines
changed function loading to use a check for function existance instead of reuqire_once
attempted remove of try catch
readded try catch
attempt to put the functions in same file to see if the performance goes up
removed the in file funciton defs
changed back to require_once
Merge branch 'master' of https://github.com/yjv/Twig