memory access for js callees

[devsnek]

reposted from WebAssembly/gc#67

It's not possible for JS callees to gain access to S.mems[0] without it being explicitly exported under a prenegotiated name. This means that it's not possible to work with functions that require memory access within the wasm start function, and it makes it impossible to import a function with the esm proposal that can access the memory.

This seems like a super important step to consider with the design of memory moving forward. I've seen it be a design blocker at least once within WASI (WebAssembly/WASI#19), and I've had to work around it in various other projects I've worked on.

The main idea so far seems to be first-class references to memories.

cc @linclark @tschneidereit @lukewagner

[sbc100]

I think this is a duplicate of #1160