Add uhashlib with sha256 implementation

Ok.  My main thought was that on 64-bit machines an int was 64-bits wide and thus the static tables would be 2 times bigger for no reason.  But, no, an int is still 32-bits, and it seems I'll keep forgetting that fact :)

But recall that 16-bit dspic was a Kickstarter pledged port, which has not yet materialised.