Move string hash and error type fields

Move string hash and error type fields

Move the string hash back into the String class.
Add a subtype field to Managed, and use that to store
the type of the ErrorObject.

Change-Id: I4aa8f1f39f512e3a9084b249e173bff41175d543
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>