Some bad assumptions were made during the creation of our Python AES GCM
code. This is now modified to be more in-line with other libraries. This
is an API breaking change on unreleased code.
This now allows for aad data to be used, varying length of
authentication tags and fixes a bug for multipart.
1. Now unified to a single class AesGcmStream()
2. Used `encrypt()` and `decrypt()` instead of `update()` to avoid
confusion over encryption and aad semantics
3. final tag_bytes is configurable in the constructor
4. `set_aad()` added to add the aad data
5. aad data is cleared after first `encrypt()` or `decrypt()` call due
to quirk in the C API.
6. More tests added
Andrew Hutchings