Add simple caching to XML compilation

XML compilation stage will perform many (expensive) resource ID
lookups - for the same entries. Adding this simple cache can
dramatically improve performance of this stage (often a bottleneck)

Change-Id: Ib1ab82ba9bf901bc2a3f9027b47d680861d6c315
Signed-off-by: Jorrit Jongma <jorrit@jongma.org>