icons-hash: misc improvements (#1553)

icons-hash: misc improvements (#1553)

* icons-hash: take total probe count into account as well

* icons-hash: use a better PRNG

the older method was using a multiplicative congruential generator (MCG)
which doesn't work too well especially with just 32 bits of state.

change it to a PCG instead with 64 bits of state (and 32 bits of output)
which should give better results.

and since we should get better rng - the search iteration has been
halved as well to save some build time.

* icons-hash: use an xor-rotate hash function

* icons-hash: fix some compiler warnings