Add faster variable freshening

Comparing with master:

$ time pts --quiet --instance uu UU/D/count.lpts

real	0m5.665s
user	0m5.574s
sys	0m0.075s
$ time ./pts-master --quiet --instance uu UU/D/count.lpts

real	0m6.063s
user	0m5.969s
sys	0m0.076s

I repeated this more often, and the results look relatively stable.

For testing, I've used
Toxaris/tsr-experiments#636a1f5f9a6c79f8becaa98486f80a3f1c00221f with
`countRebuildIncompl` commented back in.
Speed up operations with insertLookupWithKey
Merge pull request #90 from Blaisorblade/topic/performance-3

Speed up variable freshening