Add hashing processes like md5 and sha1
Test that the hashes emit at most one element.
Use a random chunk size for tests
Make the hash processes vals instead of defs
Revert "Make the hash processes vals instead of defs"

This reverts commit 9e1dc8f8d1470e26f6dd09af9585518a4f3d3e57.

This was a bad idea since the same MessageDigest could be accessed
parallel in different threads.
Produce a new MessageDigest for each process
Add and use suspend1 to get a new MessageDigest on every process run
Do not use a trait for hash
Merge pull request #124 from fthomas/hash

Add hashing processes like md5 and sha1