isEnum should also accept an enumeration object

[lazarljubenovic]

Instead of writing

export const isLanguageCode = tg.isEnum(...enumValues(LanguageCode))

where enumValues is a function defied as follows

export function enumValues<T extends object> (enumeration: T): Array<T[keyof T]> {
  return objectKeys(enumeration)
    .filter(k => isNaN(Number(k)))
    .map(k => enumeration[k])
}

allow the consumer to simply write

export const isLanguageCode = tg.isEnum(LanguageCode)

Maybe deprecate listing all values and create a new guard for that?