Improve C4::Charset::MarcToUTF8Record performance

Improve C4::Charset::MarcToUTF8Record performance

A script like bulkmarkimport.pl spends most of the time
in C4::Charset::MarcToUTF8Record function, and
specifically in C4::Charset::char_decode5426
just initializing a hash. This patch moves this
hash outside function to avoid its initializing
each time the functon is called.

A test on a specific conversion script shows me
that performances were improved from 23s to 8s.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>