Speed up Str.trans

It does this by removing matchers that have no chance
of matching again.

JSON::Tiny::to_string uses

$d.trans(['"',  '\\',   "\b", "\f", "\n", "\r", "\t"]
              => ['\"', '\\\\', '\b', '\f', '\n', '\r', '\t'])

With $d = "\n" x 10000, this patch reduces the run time from 13.5s
to 4.9s.