Add SocketUser.MutualGuilds + various command ext. methods. (#1037)

* Add SocketUser.MutualGuilds + various ext. methods.

* Search through submodules for GetExecutableCommandAsync

* Allow GetExecutableCommandsAsync(ModuleInfo) to recurse properly to all submodules.

* Bump down lang. version & whitespace cleanup.

* Change to use Task.WhenAll

* Change to ICollection<CommandInfo>

* Resolve build errors.

src/Discord.Net.Commands/Extensions/CommandServiceExtensions.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    public static class CommandServiceExtensions
+    {
+        public static async Task<IReadOnlyCollection<CommandInfo>> GetExecutableCommandsAsync(this ICollection<CommandInfo> commands, ICommandContext context, IServiceProvider provider)
+        {
+            var executableCommands = new List<CommandInfo>();
+
+            var tasks = commands.Select(async c => { var result = await c.CheckPreconditionsAsync(context, provider).ConfigureAwait(false); return new { Command = c, PreconditionResult = result }; });
+
+            var results = await Task.WhenAll(tasks);
+
+            foreach (var result in results)
+            {
+                if (result.PreconditionResult.IsSuccess)
+                    executableCommands.Add(result.Command);
+            }
+
+            return executableCommands;
+        }
+        public static Task<IReadOnlyCollection<CommandInfo>> GetExecutableCommandsAsync(this CommandService commandService, ICommandContext context, IServiceProvider provider)
+            => GetExecutableCommandsAsync(commandService.Commands.ToArray(), context, provider);
+        public static async Task<IReadOnlyCollection<CommandInfo>> GetExecutableCommandsAsync(this ModuleInfo module, ICommandContext context, IServiceProvider provider)
+        {
+            var executableCommands = new List<CommandInfo>();
+
+            executableCommands.AddRange(await module.Commands.ToArray().GetExecutableCommandsAsync(context, provider).ConfigureAwait(false));
+
+            var tasks = module.Submodules.Select(async s => await s.GetExecutableCommandsAsync(context, provider).ConfigureAwait(false));
+            var results = await Task.WhenAll(tasks);
+
+            executableCommands.AddRange(results.SelectMany(c => c));
+
+            return executableCommands;
+        }
+    }
+}