When diffing snapshots, we don't need to include changes in either the wasm stack or thread stacks (as these are only useful to the locally executing thread(s)).

To enforce this, this PR adds ignore regions to the underlying snapshots covering both stack regions.

Changes:

• Add ignore regions to snapshots before executing wasm modules (this is the only time we know both where the stacks will be, and which snapshot will be used).
• Add test to check that diffs in the stacks are ignored, but not the heap.