computePermissionsFor method
Compute member's permissions in this 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> computePermissionsFor(PartialMember member) async =>
await computePermissions(this, await member.get());