Dedupe some utility code

1. convertToArray is a duplicate of arrayFrom
2. inferFromParameterTypes delegates immediately to inferFromTypes
3. One usage of arrayFrom instantiated a whole iterator only to take the
first element, which is the same as calling `next`.

src/compiler/commandLineParser.ts

@@ -1290,7 +1290,7 @@ namespace ts {
                 case "object":
                     return {};
                 default:
-                    return arrayFrom((<CommandLineOptionOfCustomType>option).type.keys())[0];
+                    return (option as CommandLineOptionOfCustomType).type.keys().next().value;
             }
         }