Streaming decryption support

Right now `AES.decrypt` wants to have all bytes available before it starts, which isn't very practical for possibly large payloads.

Start by examining the AES.decrypt entry point mentioned in the issue and determine how it currently consumes the complete byte sequence. The issue names no files or tests, so trace its implementation and existing decryption tests before defining the streaming interface. Done means large payloads can be decrypted incrementally rather than requiring all bytes up front.

Written by the indexing model from the issue text.