std.hash design

[Johannes Pfau]

Here's a first proposal for the API:
http://dl.dropbox.com/u/24218791/d/src/digest.html

One open question is:
What should we do if a too small buffer is passed to the finish
function (in the OOP API)?

Should we check for the length only in debug(assert) or in
debug+release mode (enforce) or should we use the tango way and
silently allocate? 

And another question: Can Digest.length be pure (for all digests)?