TransliterationImpl::transliterate() has a slow-path for the case when
more than one trasliteration module is cascaded which swaps 2
uno::Sequence.  This is unbelievably slow because non-const
Sequence::operator[] does a function call into cppu to check whether COW
has to be done.

This speeds up transliterate() from 344 billion to 101 billion callgrind
cycles when built with GCC 4.9.2 -m32 -Os.

Commit d2771b63 added a second
transliteration module that is enabled by default, making the problem
visible, especially with long paragraphs in Writer.

Change-Id: I2799df9173ac73aab8c4eb4cc6f592976b06c8da