By switching to our DenseMap/Set fork with that allows parameterizing
on this.

This is the same number that CPython uses.  We were previously
using llvm's default of 64, which is pretty high -- probably fine
for their use cases, but in Python programs with lots of sets/dicts
we spend a lot of time doing malloc() for all those 1kB+ allocations.
This also increases the time it takes to iterate over the dict/set,
since there are more empty buckets that have to be read + skipped.