Add Multi-encrypt functions

[jagerman]

This allows efficiently encrypting 1-to-N or N-to-N messages in a single message, such as is used in the group keys message.

This adds both a general C++ interface (which is now used by the group keys code) for constructing such encrypted messages generically, and adds a version suffixed with _simple that produces ready-to-go bt-encoded binary values containing the nonce + encrypted values.

[jagerman]

Merging this as it's being used without any apparent issues now in dev versions of Session.