Detect contiguous unused elements in pools and introduce ranges for free element spots.

[dimonkov]

• Detect when items in LinearPool::freeItems are touching and introduce free item ranges.
• Compact (collapse) adjacent free ranges that are touching into a single range, example [0, 43] and [44, 59] -> [0, 59].
• Possibly look into storing free ranges as a sorted tree structure to help with compacting.

struct ItemRange
{
    uint32 begin = 0;
    uint32 end = 0;
};

It can help to reduce freeItems array memory usage.