perf: switch to ska::flat_hash_map instead of std::unordered_map (#30)

* perf: switch to ska::flat_hash_map instead of std::unordered_map

Small ~10% improvement to connectomics.npy renumber, but ~45%
improvement to a random array.

* fix: support 32-bit

skarupke/flat_hash_map#18

* fix: allocate default table

Avoids problems with dynamic library situations:
skarupke/flat_hash_map#26

* fix: use more compatible definition of void_t

* docs: update comment date