Simple cache of url for QQmlCompiledData

Removes URL parsing and allocation/deallocation overhead as a bottleneck from
cached instantiation. Takes total runtime percentage of beginCreate from 6208ms
to 4987ms out of a 15 second trace of the instantiation_cached librarymetrics
benchmark with the empty Item data.

Task-number: QTBUG-43096
Change-Id: Ie4ec8e83461b4926e502dd78a7178cc8e8131e70
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>