object is NULL. If the blob's length is 0 or its priv pointer is NULL,
pass over it silently -- not an error, but no need to call the hash
update.

allow the .final() method to be called more than once, caching the result
on the first call

This is still WIP, has a number of todos and needs more tests.

This reverts commit 81bd32a0.

Restore m4/PLACEHOLDER, so that the m4 directory exists