enum (value, schema, opts) {
    const possibleValues = schema.enum
    if (utils.findIndex(possibleValues, (item) => utils.deepEqual(item, value)) === -1) {
      return makeError(value, `one of (${possibleValues.join(', ')})`, opts)
    }
  }
Validates the provided value against an array of possible values defined by the Schema's `enum` keyword
Validation succeeds if the value is deeply equal to one of the values in the array.
see http://json-schema.org/latest/json-schema-validation.html#anchor76

@name Schema.validationKeywords.enum
@method
@param {*} value Value to validate
@param {object} schema Schema containing the `enum` keyword.
@param {object} [opts] Configuration options.
@returns {(array|undefined)} Array of errors or `undefined` if valid.