Support using Enum and UnitEnum implementations in BSON

[thekid]

This pull request makes XP enums and PHP enums usable in the MongoDB API. Both are stored as strings using their names.

Example:

use com\mongodb\{MongoConnection, Document};
use util\Currency;

enum Coin : int {
  case Penny = 1;
  case Nickel = 5;
  case Dime = 10;
  case Quarter = 25;
}

// We can now directly use it in queries, criteria, pipelines etc.
$conn= new MongoConnection('...');
$assessment= $conn->collection('inventory.users')->insert(new Document([
  'owner'    => $user,
  'coin'     => Coin::Penny, 
  'currency' => Currency::$USD,
]));

This is consistent with marshalling in this library.