mbsync: write function to generate group-channel blocks

This function takes in a set of groups, and their consituent
channels and writes the appropriate .mbsyncrc block. The block is as
shown below:

      Group groupName1
      Channel channelName1
      Channel channelName2

      Group groupName2
      Channel channelName3

Each group must have a unique name, no matter which account it is
declared under. The same holds true for channels. However, if there is
a group that shares the same name as the channel, the channel will
effectively be "shadowed" by the group, and mbsync will default to
working with the group in that case.

modules/programs/mbsync.nix

@@ -78,6 +78,27 @@ let
         Expunge = masterSlaveMapping.${mbsync.expunge};
         SyncState = "*";
       } // mbsync.extraConfig.channel) + "\n";
+  # Given the attr set of groups, return a string of channels to put into each group.
+  # Given the attr set of groups, return a string which maps channels to groups
+  genAccountGroups = groups:
+    let
+      # Given the name of the group and the attribute set of channels, make
+      # make "Channel <grpName>-<chnName>" for each channel to list os strings
+      genChannelStrings = groupName: channels: mapAttrsToList
+        (name: info: "Channel ${groupName}-${name}") channels;
+      # Take in 1 group, construct the "Group <grpName>" header, and construct
+      # each of the channels.
+      genGroupChannelString = group:
+        [("Group " + group.name)] ++
+        (genChannelStrings group.name group.channels);
+      # Given set of groups, generates list of strings, where each string is one
+      # of the groups and its consituent channels.
+      genGroupsStrings = mapAttrsToList (name: info: concatStringsSep "\n"
+        (genGroupChannelString groups.${name})) groups;
+    in (concatStringsSep "\n\n" genGroupsStrings) # Put all strings together.
+       # 2 \n needed in concatStringsSep because last element genGroupsStrings
+       # has no \n.
+       + "\n\n"; # Additional spacing after this account's group setup.
 
   genGroupConfig = name: channels:
     let