Counter mode

Additional source code for the CTR mode.

Formal

What's Changed

• Support formal verification with CBMC;
• Update signatures for public functions to return call results.

The basic edition.

This is a lightly revamped variant of the legacy source code from http://www.literatecode.com.

What's Changed

• Source code minor touch-up and restyle;
• Fixed a few pedantic warnings from clang/GCC;
• Modified demo to use multiple test vectors.