> - make dicts denser by making the resize factor 2 instead of 4 for small dicts

This had been implemented already when I start compact dict.

> - improve cache locality on collisions by using linear probing

set does this. But dict doesn't do it for now.

In case of compact dict, liner probing only affects index table (dk_indices).
dk_indices is small (64byte when dk_size==64). One or two cache line
can contain whole dk_indices of small dicts.
So performance benefit of linear probing will be smaller than previous
dict implementation.

I'll re-evaluate it.