Introduce flag to prevent reallocation of shared memory

The issue names no repository files, tests, or implementation entry points; start by reviewing the WebAssembly threads proposal and the shared-memory instantiation examples in the report. Establish whether a module-associated memory cache or flag is within scope, define its lifetime and encapsulation semantics, and document the compatibility and validation criteria before implementation.

Instantiating a module more than once allocates new memory for each instance, what if, instead, there could be a flag that would cause all instances to refer to the same memory?
Instead of them being thread-local memories, they would be "global."

This allows Wasm to remain completely self-contained, encapsulated, and in control of its own memory.
Currently, shared memory is completely useless if not importing host shared memory, but every step in between the module getting its memory, and the host passing it to the module, leaves room for mutation, and unexpected change at runtime, as the host has control over the array buffer too.

Nothing more would need to be changed, it would just change the lifetime of the memory.

This could be implemented as a simple cache associated with the module.

Current loading (JS):

// main.mjs:
const memory = new WebAssembly.Memory({ ..., shared: true });

const {
    instance,
    module
} = await WebAssembly.instantiateStreaming(..., ...memory...);

const thread = new Worker("./worker.mjs", ...);

worker.postMessage({ memory, module });

// worker.mjs:
... // await .postMessage({ memory, module })
const instance = await WebAssembly.instantiate(module, ...memory...);

With cached memory:

// main.mjs:
const {
    instance,
    module
} = await WebAssembly.instantiateStreaming(...);

const thread = new Worker("./worker.mjs", ...);

worker.postMessage(module);

// worker.mjs:
... // await .postMessage(memory)
const instance = await WebAssembly.instantiate(module);

In the former example, the WebAssembly module's memory may be corrupted later at the will of the host, breaking encapsulation, but the way that multi-threaded Wasm loading works, it leaves no alternative.

Would this break any of the core motivations/ideas/philosophies behind WebAssembly?