Speedups

These commits implement the suggestions in https://jira.mongodb.org/browse/PERL-129, for a 65% speedup for inserts and 14% speedup for fetching (using the insert.pl script in the ticket).
FYI: I don't know if this code is still current - but converting calls from `newSVpvn` to `newSVpvs` is just going to have the latter doing the call to `strlen`. What you want to be doing is using `newSVpvn` and telling it the exact length (which is easily computable here because they're all constant strings).