Add an implementation of the SHA1 hash algorithm based on a public
domain project by Steve Reid <email@example.com>. You can get the
original sources from:
We will need this for several things. File names of note files will be
generated based on a hash their content instead of using a random name.
Items can be uniquely identified using a hash.
In addition to the regular sha1_init(), sha1_update() and sha1_final()
operations, our implementation also contains wrappers for hashing a
string or a stream.
Signed-off-by: Lukas Fleischer <firstname.lastname@example.org>