address review comments

Author: jonsequitur

src/System.CommandLine/Parsing/ArgumentResult.cs

@@ -51,7 +51,7 @@ internal virtual ArgumentConversionResult Convert(
             var parentResult = Parent;
 
             if (ShouldCheckArity() &&
-                parentResult is {} &&
+                parentResult is { } &&
                 ArgumentArity.Validate(parentResult,
                                        argument,
                                        argument.Arity.MinimumNumberOfValues,

src/System.CommandLine/Parsing/StringExtensions.cs

@@ -566,7 +566,7 @@ private static Dictionary<string, Token> ValidTokens(this ICommand command)
 
             for (var commandAliasIndex = 0; commandAliasIndex < command.RawAliases.Count; commandAliasIndex++)
             {
-                var commandAlias = command.RawAliases.ToArray()[commandAliasIndex];
+                var commandAlias = command.RawAliases.ElementAt(commandAliasIndex);
 
                 tokens.Add(
                     commandAlias,
@@ -580,7 +580,7 @@ private static Dictionary<string, Token> ValidTokens(this ICommand command)
 
                     for (var childAliasIndex = 0; childAliasIndex < child.RawAliases.Count; childAliasIndex++)
                     {
-                        var childAlias = child.RawAliases.ToArray()[childAliasIndex];
+                        var childAlias = child.RawAliases.ElementAt(childAliasIndex);
 
                         tokens[childAlias] =
                             new Token(