Change signature of isType to allow any

There's no need to restrict the type of argument being tested. In
particular, this is convenient for nullable strings.

Author: dphilipson

README.md

@@ -348,7 +348,7 @@ unnecessary for most users. There are still a few niche reasons why users may
 still prefer to use this library.
 
 * This library provides several helper functions which cannot easy be
-  implemented for native enums. Of these, `Enum.isType()` will likely be the
+  implemented for native enums. Of these, [`Enum.isType()`](#enumistypeenum-value) will likely be the
   most useful.
 * Defining a native string enum involves a bit of repetition, as each value must
   be written twice:

src/index.ts

@@ -46,7 +46,7 @@ export namespace Enum {
 
     export function isType<
         T extends { [_: string]: any }
-    >(e: T, value: string): value is Enum<T> {
+    >(e: T, value: any): value is Enum<T> {
         return values(e).indexOf(value) !== -1;
     }
 }