In some places, we store a member of a class as std::unique_ptr<T> even if T is not polymorphic. 

On hot path, this needless memory indirections can affect CPU cache and accumulate to some performance losses.

We need to test if there are some performance gains hidden in this.

There are a lot of cases in SBE where we store std::unique_ptr<vm::CodeFragment> instead of just vm::CodeFragment.
https://github.com/search?q=repo%3Amongodb%2Fmongo%20std%3A%3Aunique_ptr%3Cvm%3A%3ACodeFragment%3E&type=code