Identify unnecessary clones, copies, and general inefficiencies

[radu-matei]

As we approach https://github.com/fermyon/spin/milestone/1, we should revisit the entire codebase and optimize it where possible.

[radu-matei]

There have been several passes over the implementation of the Spin internals, and #738 should address even more.

As this is not currently an issue with Spin, I am closing this. We could re-open in the future if we identity issues here.