[x/program] Rename SmartPtr type

[samliok]

          Do you want to create an issue to rename this? Smart-pointers are already thing in Rust [^1]. An `Arc` is an example of a smart-pointer. While there are tons of terms that are overloaded, I wouldn't want a new program dev to land in this code for whatever reason and get confused. 

Originally posted by @richardpringle in #623 (comment)

Potential names MemoryPtr, or ArgumentPtr, ArgPtr...

[richardpringle]

As mentioned offline, I'm good with pretty much any name. HostPtr might be another option since that's essentially what we're doing here.

[github-actions]

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