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).
Per my other comment - this is not faster. What's fastest is `hv_fetch(hv, "_id", 3, 0)`.

It's also faster to simply do the `hv_fetch` and use the result of that to determine if the key exists rather than using `hv_exists` right before it. They both do similar things to compute the hash and do the table lookup. Take advantage of that by getting rid of `hv_exists`.