Introduce an Identifier class

Introduce an Identifier class

Identifier are now (opaque) pointers. They contain the
string and it's hash value making conversion from Identifier
back to it's string value a no-op.

Change-Id: Ied7e845a981514890ff6a686e0e3e3057e6b6fbf
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>