Crypto: Add large input tests for all Digests

Create a helper function in cryptoutil.rs which feeds 1,000,000 'a's into
a Digest with varying input sizes and then checks the result. This is
essentially the same as one of Sha1's existing tests, so, that test was
re-implemented using this method. New tests were added using this method for
Sha512 and Sha256.
Crypto: Add overflow checking addition functions.

Added functions to cryptoutil.rs that perform an addition after shifting
the 2nd parameter by a specified constant. These function fail!() if integer
overflow will result. Updated the Sha2 implementation to use these functions.
Sha1: Update Sha1 to use new functionality in cryptoutil.rs