Memory-safe nouns outside of NockStack (replace NounSlab'ish)

I have a prototype for this, it would functionally be a replacement for the `NounSlab` type in https://github.com/zorp-corp/nockchain

The _very_ unbaked sketch idea I had for this lives at https://github.com/zorp-corp/nockchain/tree/master/crates/nockvm/rust/nockvm/src/substantive

Start by reading the NounSlab type in the nockchain repository and the sketch under crates/nockvm/rust/nockvm/src/substantive. The issue does not name tests, entry points, or the required behavior, so the first step is to establish the replacement's scope and memory-safety requirements. Done would mean an agreed, implemented replacement for NounSlab outside NockStack.

Written by the indexing model from the issue text.