PERF: hashtable for 16, 32, 128 bit dtypes
@jorisvandenbossche has pointed out in a couple of threads recently that the hashtables we use only support 64 bit float/uint/int dtypes.  As a result, whenever we need to factorize a smaller dtype, we have to copy+upcast.

khash.pxd looks like it has int32 functions; might there be uint32 or float32 variants available somewhere in the C code?  If not, what would it take to implement them?  cc @WillAyd @chris-b1 