Differing enum values

[kourge]

This fixes #5. It is more or less a merging of the original implementation of this module and the example provided by @weswigham. The problem exhibited by WeirdEnum is solved by giving the function two type parameters, both of which are inferred by the TypeScript compiler. Additionally, the forEach call was replaced with for-of to allow this module to work even in an ES3 environment with no need for an ES5 Array.prototype.forEach polyfill.

There is still some room for improvement:

• TypeScript does not prohibit you from applying keyof to a non-object type. For instance, given type Enum<T> = T[keyof T], one can write Enum<number> and not fail compilation. (The result is harmless, although meaningless: number.) If we raise the TypeScript dependency of this module from 2.1 to 2.2, we can use the object type as a constraint: type Enum<T extends object> = T[keyof T].
• Introduce type-safe variants of Object.keys and Object.values to be used with these enum types. For example, given const e = {a: 'A', b: 'B'}, Enum.keys(e) should yield ('a' | 'b')[] (instead of string[] like Object.keys does) and Enum.values(e) should yield ('A' | 'B')[] (instead of any[] like Object.values does).

I believe this also solves #7, so long as you define an enum using the extended form:

export const Status = Enum({
  /**
   * The service is running and operational
   */
  RUNNING: 'running',

  /**
   * Something is wrong
   */
  STOPPED: 'stopped',
});
export type Status = Enum<typeof Status>;

Rather than the short form:

export const Status = Enum('running', 'stopped');
export type Status = Enum<typeof Status>;

[dphilipson]

This is fantastic, thank you very much for this contribution!