Support an `actor` mode for pipelines

This mode would enable working with pipelines following the [Actor Model](https://en.wikipedia.org/wiki/Actor_model) in addition to use as a pure function.

For Emscripten:

- Explicitly export `stackSave`, `stackRestore`, per https://github.com/emscripten-core/emscripten/commit/cc1112ad32d13a67fe2534a4e47e3177e736f95b
- Add an `actor: bool` to `runPipeline`'s options.
- Call `stackSave` / `stackPtr` per https://github.com/Kitware/VolView/pull/281/commits/7690cde3c3d12bf78fd096b7a5b604d55d5fdc96

For WASI:

- And an `actor: boolean` option to `run_pipeline`
- Return an `actor` for re-use -- the last positional output? The wasm runtime `store`?
- Do not run destructor's at the end of execution.
- Run `stackSave` and `stackRestore`?