Discussed with@alcaeus at SymfonyCon, this improves support for partial-initialization of lazy ghost objects.

• isLazyObjectInitialized(bool $partial = false): bool allows checking if an object is partially-initialized.
• a partial initializer set to the special"\0" key can be used to initialize all/many properties at once.

Targeting 6.2 because this would be a BC break in 6.3, also because this improves a new unreleased feature.