The current implementation of the G1 sparse remembered sets we use CardIdx_t as elements for SparsePRTEntrys, which are ints, while card offsets within a region can only range [0, 0xffff], since regions are capped at 32 MB.

The reason for this is that -1 is used to represent null value entries in SparsePRTEntry.
A key observation is that all the null entries are at the end of the array in SparsePRTEntry,
which means that adding an integer field _next_null, will be sufficient to keep track of the null entries.

This also means that clearing an array can be made faster, since it just involves setting this integer.