latest-assert-build

CogVM source as per VMMaker.oscog-eem.3500

Spur:
Fix the new hash generator introduced in VMMaker.oscog-eem.3485.
That version used a 32-bit or 64-bit shift register and

• an object's hash could end up as zero since the least significant 22 bits of
  the counter could be zero even if the counter was not (a bad bug!)
• the counter would take longer than 2^22 - 1 iterations to cycle through all
  possible 2^22 - 1 hash values since again the counter would take either
  2^32-1 or 2^64-1 iterations to cycle through all possible 32 or 64 bit values

Replace it with a 22 bit counter that cycles through all 2^22 - 1 possible
non-zero states in 2^22 - 1 iterations. Thanks to Juan Vuletich for the code.

Fix bug in primitiveSetOrHasIdentityHash on Spur where when setting the hash
the primitive would return a new hash value rather than the raw hash bits.

Don't cycle the identityHash PNG on writing a snapshot. Just write its value
direct to the image file.

V3: Fix bug where the hash seed saved to the image would be mre or less
constant because it was derived from freeStart and after a garbage collection
freeStart would be reset. So write the value of freeStart to lastHash before
an incrementalGC and write that to the image header.