Import custom memory manager?

Per [Emscripten Compiler Settings ](https://emscripten.org/docs/tools_reference/settings_reference.html#malloc), it is possible to omit any malloc implementation and provide your own. To reduce module size and improve efficiency, I would like to import `malloc` and `free` implementations; that way, modules don't have to all have their own copy of the allocator code, and heap overhead can live outside of WASM memory.

However, I can't find any way to safely do this. A custom allocator needs to know how Emscripten has laid out memory, and I can't find any options that will force it to export or import `__heap_base`, or documentation for how stack and heap memory are ordered, etc.

So, are there any examples of how to implement custom imported memory management functions?