Consider storing 6 bits of key hash codes in tagged pointers

[puzpuzpuz]

Since 64-bit pointers in Go are 8-byte aligned, we could use free 2x3 bits to store a (MSB?) part of the key hash code as tagged pointers in key and value pointers. This should improve search performance since this way we won't need to calculate equality of all scanned keys, but only a part of them.