[host/mem] Review `SharedMemory` trait

We should review our APIs related to interacting w/ shared memory. In particular:
- `as_mut_slice`,
- `copy_from_slice`, and
- `copy_to_slice`.

These APIs allow read/writes to any shared memory offset. We should create higher-level abstractions to write to specific/named offsets.

Start by locating the SharedMemory trait and its as_mut_slice, copy_from_slice, and copy_to_slice APIs, then review how callers use arbitrary offsets. Define and validate higher-level abstractions for specific or named offsets; the issue does not name tests or files, so the acceptance criteria need clarification.

Written by the indexing model from the issue text.