fix module inclusion issue, corrected/updated benchmarks

move I18nema::Backend#translate into a module, so that it can be
overridden by other modules (e.g. I18n::Backend::Fallbacks)

previous benchmarks were incorrect, due to inadvertant inclusion of
I18n::Backend::Fallbacks in the I18n tests.

Change-Id: Ie4e4c7f3c2b58c9e95a8b0d563b3caf69e17c9d0