For reasons lost to time, the extension contains classes named CursorId and ObjectID. For consistency, we should rename ObjectID to ObjectId.

This will be consistent with the legacy extension's "MongoId" class, and existing method names (e.g. getCursorId(), getUpsertedId()). Acronyms such as UTCDateTime should remain capitalized (see: PDOStatement).

Class names in PHP are case-insensitive, so this is not a BC break.