Add FFI API botan_cipher_requires_entire_message()

[reneme]

... as it says on the box.

@randombit One question: Should this be added to the python wrapper? I'm unsure, given that the python module just collects whatever shared object it finds. What happens, if it get a hold of an older library version whose FFI didn't provide this method. What's our contract here?

[randombit]

Should this be added to the python wrapper? I'm unsure, given that the python module just collects whatever shared object it finds. What happens, if it get a hold of an older library version whose FFI didn't provide this method. What's our contract here?

The Python wrapper checks that botan_ffi_supports_api accepts the version it is hardcoded with, matching the date code of BOTAN_HAS_FFI.

Since this adds a new FFI interface we should

• Update the version code in ffi/info.txt
• Update botan_ffi_supports_api
• Update botan3.py to use the new version
• Add the FFI decl to botan3.py ideally also with a native Python call

We failed to do this correctly in 3.3.0 which added an FFI interface. I'll update the docs about this topic since they do not address these issues at all.

[reneme]

Updated accordingly. I took the liberty to also add ideal_update_granularity and make is_authenticated a wrapper around botan_cipher_is_authenticated.

[randombit]

Looks great thanks!

[coveralls]

coverage: 92.082% (+0.002%) from 92.08%
when pulling 98834c9 on Rohde-Schwarz:feature/ffi_cipher_requires_entire_message
into b059347 on randombit:master.