Implements ArrayAccess

[roberto-butti]

In order to access to Arr object like an array:

• Add implements ArrayAccess
• implements new methods specific for ArrayAccess:

  public offsetExists(mixed $offset): bool
  public offsetGet(mixed $offset): mixed
  public offsetSet(mixed $offset, mixed $value): void
  public offsetUnset(mixed $offset): void

[roberto-butti]

closed with #2