Fix interpet's use of stale memory

[krtab]

Each time the memory is cloned, previous versions should be considered stale. This is especially true for #433 where the inner data structures are made purely functional, but is likely to cause bugs even without.

We should think of better ways to ensure this invariant is not broken, as the bugs caused are pretty nasty to track down.

[zapashcanon]

There are three cases:

• we have to add/move the let* mem after cloning, this is needed to be correct;
• we simply move down the let* mem even without cloning, this is more future proof;
• we add a new let* mem even when there's no cloning, this is more future proof but it also makes things slower.

I'm fine about keeping things as is but I also think we could remove the last case?

I'll let you choose what you prefer and then I'll merge.

[zapashcanon]

Fine after discussing!

(The `let> is actually hidding a select)