transition Permissions from u64 to SmolBitSet

[fgardt]

The overdue integration of smolbitset to prepare for the future breakage when discord goes past 64 permissions.

Things compile but I've not tested it yet.

Documentation still needs to be updated and I'm a bit unsure about the associated constants on Permissions since those will not be possible to make (constant) for future permissions >= 63.
The Permission enum is a decent enough alternative I think but probably should also implement the usual bit ops and return Permissions.