Reduce memory overhead

[nikic]

Currently each cache entry has about ~256 bytes overhead (cache entry, cache slot, pool, pool block, key string header). Small pools are currently allocated in 512 byte increments, so the minimum entry size is 512 bytes.

There is clearly some optimization potential here...

[nikic]

First optimizations implemented in #299, saving 32 bytes per entry.

[nikic]

Two more optimizations have been implemented in #300 and #301, saving another 32 bytes.

[nikic]

Between other changes, #302 saves another 8 bytes.

[nikic]

#305 implements some major memory savings. Depending on the case they range between a couple percent to a factor of three.

[nikic]

Closing this, as the major work happened in #305. There are still improvements that can be made, but it's peanuts in comparison.