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).
As the current maintainer for MongoDB, I'm happy to get optimization suggestions against the **current** codebase, but I should caution that over the the next several months, I'll be factoring out the BSON code to a separate module and harmonizing it with the pure-Perl BSON module so we can have the same sort of pure-Perl or XS-optimized implementation choices as YAML or JSON.  That split out will be the best time for a review of the codebase and optimization suggestions.