Add support for AES CFB8 cipher

This is mostly backporting the CFB8 code in nettle's master tree so we can use it also when compiled against 3.4

Fixes #357 (closed)

Checklist

• Code modified for feature
• Test suite updated with functionality tests

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code