Integrate new io.Blob class

[thekid]

This pull request integrates the new io.Blob class into the JSON output layer.

Example

use io\Blob;
use text\json\Json;
use util\cmd\Console;

$in= Console::$in->stream();
$out= Console::$out->stream();

// This writes the data while reading it
Json::write(['data' => new Blob($in)], $out);

[thekid]

This requires XP 12.9+ but composer.json still reads:

  "require" : {
    "xp-framework/core": "^12.0 | ^11.0 | ^10.0",
    "php" : ">=7.4.0"
  },

We could backport this class to XP 11; or we rewrite the tests to work without the Blob class.