This reverts commit 865796c3687cc06920ddf29890de991287dc50f6.

(Superseeded by next commit)

Import hashlib if it is there, otherwise import md5.  Adds method
file_digest that wraps the logic on which module to invoke.

This adds a module that concatenates (ie. assembles) a file from
fragments in a directory in alphabetical order.  It chains the file
module afterward to fix up ownership and permission.  This also adds
tests for the assemble module with fragments in assemble.d.