RFR JDK-8047328: Change typedef CardIdx_t from int to uint16_t

[Andreas Sjöberg]

Hi all,

could I please have reviews for this patch that changes the typedef 
CardIdx_t from int to uint16_t. The motivation behind this patch is to 
reduce the memory footprint caused by the G1 remembered sets.

This adds a _next_null field to the SparsePRTEntry class which keeps 
track of where the next possible insert could be. The other 
modifications are to make use of the fact that we know exactly how many 
cards are contained in the SparsePRTEntry, and no longer have to compare 
against a designated NullEntry value.

webrev: http://cr.openjdk.java.net/~jwilhelm/8047328/webrev/

Thanks,
Andreas