Permissons should be using a bigint instead of u64

[GnomedDev]

The permissions type is documented on ddev docs to say:

Permissions are stored in a variable-length integer serialized into a string, and are calculated using bitwise operations. For example, the permission value 123 will be serialized as "123". For long-term stability, it's recommended to deserialize the permissions using your preferred languages' Big Integer libraries.

This was also echoed by discord staff in the large bot channels a while ago, as they are coming up to 64 permissions. This will be a breaking change as bitflags is not compatible with bigints, to my knowledge, and we wouldn't want to use a naive bigint implementation anyway for performance reasons.