Additional motivation: efficient memory mapping

An application may want to map en external buffer and read/write to it directly.

A concrete example is mapping a webgl vertex buffer into a wasm module to have it filled out based by decoding a model file.

If a non-web encoding was able to mark a region as volitile, memory-mapped I/O based device drivers could be implemented in WebAssembly. (If this isn't currently possible at this late state of proposal, I'm likely to add this to a new proposal.)