Improve error messages for shape and vec -like types

[veewee]

Given:

use Psl\Type;
$shape = Type\shape([
    'name' => Type\string(),
    'articles' => Type\vec(Type\shape([
        'title' => Type\string(),
        'content' => Type\string(),
        'likes' => Type\int(),
        'comments' => Type\optional(Type\vec(Type\shape([
            'user' => Type\string(),
            'comment' => Type\string()
        ]))),
    ]))
]);

try {
    $shape->coerce([
        'name' => 'ok',
        'articles' => [
            [
                'title' => 'ok',
                'content' => new stdClass,
            ]
        ]
    ]);
} catch (Type\Exception\CoercionException $e) {
    echo $e->getMessage().PHP_EOL;
    var_dump($e->getTypeTrace()->getFrames());
}

This results in:

Could not coerce "stdClass" to type "string".

array(3) {
  [0] =>
  string(20) "array{'articles': _}"
  [1] =>
  string(120) "vec<array{'title': string, 'content': string, 'likes': int, 'comments'?: vec<array{'user': string, 'comment': string}>}>"
  [2] =>
  string(19) "array{'content': _}"
}

Both the path and frames are rather confusing and it is unclear what failed exactly without processing this information.

Suggestion:

Keep track of an additional path on the TypeTraceso that we can build errors like:

Could not coerce "stdClass" to type "string" at path articles.0.content

The path can contain a list of strings to identify the path - which will be set for shape and vec-like types:

• Shapes will mark the property: prop
• Vecs will mark the index: 0
• Dicts will mark the named index: key

The list of paths will be .-separated

❗Consider the performance/memory impact, because PSL assertions are already quite runtime-heavy