eliminate per-iterator heap allocation by constructing InternalKeyComparator in-place

[zaidoon1]

resolve 13951

[zaidoon1]

cc @cbi42 thoughts on this fix?

[andrew-kryczka]

This could conclude the saga of #6944 -> #7149 -> #9611 -> ?

Just need to make sure the InternalKeyComparator overhead is negligible (looks like just the optional could make it use more memory than the pointer).

Kind of tangential - #13951 is narrowly focused on heap allocations. In the code here size of BlockIter objects has historically been more significant. So measuring overall performance would be better.

[zaidoon1]

Just need to make sure the InternalKeyComparator overhead is negligible (looks like just the optional could make it use more memory than the pointer).

how should i go about doing this?