computePermissionsIn method
Compute this member's permissions in channel.
This method returns the permissions for member according to the
permissions granted to them by their roles at a guild level as well as
the permission overwrites for channel.
Adapted from https://discord.com/developers/docs/topics/permissions#permission-overwrites
Implementation
Future<Permissions> computePermissionsIn(GuildChannel channel) async =>
await computePermissions(channel, await get());