Avoid enormous type name + related refactoring

[jplatte]

Got nerd-sniped into looking for & fixing a huge type name by #414 (comment).

[jplatte]

The original type name contained the tuple 21 times, and the tuple itself had a type name length of 566 bytes. I think the new type name will still be a bit long (matrix_sdk_crypto::session_manager::group_sessions::GroupSessionManager::encrypt_session_for::EncryptResult, if I got that right, is 107 bytes) but the reduction in the overall type name of that drop_in_place instantiation should still be pretty good: (566 - 107) * 21 = 9639, almost cutting the length down by two thirds.