Support an `actor` mode for pipelines

[thewtex]

This mode would enable working with pipelines following the 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?

[floryst]

Should we expect pipelines operating in such fashion to call into main()? Or should we ccall into some other entrypoint we define? The former handles allocating args, while I'm not sure about the latter.

[thewtex]

Yes, actors can allocate their own internal memory and manage their own internal state. For reasons of efficiency and safety, inputs and outputs are allocated, "lowered" and "lifted" by itk-wasm bindgen or, eventually, wit bindgen.

[floryst]

Update: I encountered the stack leak behavior in itk-wasm@1.0.0-b.141 when calling readImageBlob many times using the same webworker. I fixed it by patching the itk-image-io files to call stackSave/stackRestore inside callMain(). I haven't evaluated this yet in VolView.