Support using Stringable implementations in BSON

[thekid]

This pull request makes Stringable implementations usable in the MongoDB API.

Example:

use com\mongodb\MongoConnection;
use lang\IllegalArgumentException;

// This class implicitely implements the `Stringable` interface
class Code {

  public function __construct(private string $string) {
    preg_match('/^[0-9_A-Z-]{14}$/', $string) || throw new IllegalArgumentException('Invalid code');
  }
  
  public function __toString() { return $this->string; }
}

// We can now directly use it in queries, criteria, pipelines etc.
$conn= new MongoConnection('...');
$assessment= $conn->collection('scorm.assessments')->first(new Code('...'));

This is consistent with marshalling in this library.

[thekid]

This can be dropped with the next major version requiring PHP8+