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).
No, `newSVpvs()` takes a string literal and computes the length at compile time using `sizeof()`, eliminating the need for telling the compiler something it already knows.

From `handy.h`:
```
/* concatenating with "" ensures that only literal strings are accepted as
 * argument */
#define STR_WITH_LEN(s)  ("" s ""), (sizeof(s)-1)
/* […] */
#define newSVpvs(str) Perl_newSVpvn(aTHX_ STR_WITH_LEN(str))
```

The same goes for `hv_stores()` and `hv_fetchs()` below